There are a number of complications and issues surrounding the application and use of single transferable vote proportional representation that form the basis of discussions between its advocates and detractors.

Complexity Among electorates considering the adoption of STV, there is a frequently expressed concern that PR-STV is relatively complex compared with plurality voting methods. For example, the Canadian province of British Columbia held a referendum on adopting BC-STV in 2005. When "no" voters were asked why they were voting against STV, they gave their reason as "wasn't knowledgeable".Video postmortem on BC-STV referendum (British Columbia's version of STV). The referendum resulted in a 56.16% vote in favour of adopting BC-STV, whether or not voters were any better informed, though the system was never adopted due to institutional barriers.

The use of STV is more complex than first-past-the-post but for the voter, STV is fairly simple. The vote counting under STV is simpler than primaries, which are no longer necessary if STV is used.

As with all voting systems, even if PR-STV is understood, areas of controversy surrounding its use remain. PR-STV is a form of proportional representation so issues relating to that form of electing also impacts acceptance of STV. As well, STV uses ranked voting, an unknown quantity in many places in the world.

Arguments for and against proportional representation in general are frequently referenced in debates among electorates considering STV. STV is a family of systems, and the specific implications of the use of a particular STV system is sometimes an issue.

Most of the arguments for and against STV and proportional representation in general are based around expected outcomes of the specific alternative proposed system, and not on the system itself.

Effects on parties, factions and candidates

STV differs from all other proportional representation systems in actual use in that representatives are directly elected by the voters, and candidates of one party can be elected on transfers from voters who initially gave their vote to a candidate of another party. Hence, the use of STV may reduce the role of political parties in the electoral process and correspondingly partisanship in the resulting government.

Unlike proportional representation systems employing party lists, voters in STV are not explicitly constrained by parties even when they do exist; voters may ignore candidate party labels and mix their ranked candidate preferences among parties.

Similarly, candidates may achieve electoral success by obtaining a quota of voters not generally within their own party, perhaps by winning transfers from moderates or by championing a specific issue contrary to party doctrine. STV advocates argue that, by requiring a candidate to appeal to the supporters of other candidates for their second and further preferences, it reduces adversarial confrontation, and indeed, gives a substantial advantage to candidates that broaden their appeal by being not only collegial but as open-minded and flexible in their principles as they can manage. STV detractors see that as a flaw, arguing that political parties should be able to structure public debate, mobilize and engage the electorate, and develop policy alternatives.

Unlike List PR, PR-STV can be used in elections in organizations without any political parties at all, such as in municipal non-partisan elections, trade unions, clubs, and schools.

Some practices overlaid on PR-STV, however, may encourage the role of political parties and actually strengthen them. Australian Senate elections use a combination of large districts (statewide elections), mandatory complete ballots (preferencing all candidates/full preferential voting), and compulsory voting has resulted - since they were introduced in 1983. This results in the near 95% usage of partisan group voting tickets, so political parties have significant power in determining election results by adjusting the relative ordering of their candidates on the party list.

Successful campaign strategy in PR-STV elections may differ significantly from other voting systems. In particular, individual candidates in STV have little incentive for negative campaign advertising, as reducing a particular opponent's ranking among voters does not necessarily elevate one's own; if negative campaigning is seen as distasteful by the voters, the practice may even prove harmful to the attacking candidate.

Conversely, in order to avoid elimination in early counting rounds by having too few first preference votes, candidates have a significant incentive to convince voters to rank them explicitly first as their top preference, rather than merely higher. This incentive to attain first preferences, in turn, may lead to a strategy of candidates placing greater importance on a core group of supporters. Avoiding early elimination, however, is usually not enough to win election, as a candidate must still subsequently win enough votes on transfers to achieve the quota. Consequently, strategies that sacrifice wide secondary support in favor of primary support amidst a core group may ultimately fail unless the group is large enough to have quota.

There are also tactical considerations for political parties in the number of candidates they stand in an election where full ranking on the ballots is not required. Standing too few candidates may result in all of them being elected in the early stages, and votes being transferred to candidates of other parties, instead of being used to elect candidates of the party.

Standing too many candidates might result in first-preference votes being spread too thinly amongst them, and consequently several potential winners with broad second-preference appeal may be eliminated before others are elected or eliminated and their second-preference votes distributed. The risk of such a missed opportunity is larger when voters do not stick tightly to their preferred party's candidates.

However, if voters mark their preferences for candidates from a particular party before any other candidates, then running too many candidates is not an issue. In Malta, where voters tend to stick tightly to party preferences, parties frequently stand more candidates than there are seats to be elected. Similarly, in Australian Senate elections, voters tend to vote along party lines due to the relative ease of endorsing a party slate. The parties do not encourage voters to peruse the party list but urge them to take it on trust, as evidenced by their how-to-vote cards distributed outside polling booths. Trusting in the process, voters generally do not individually cast their own unique ranking of preferences.

In the Republic of Ireland, the main political parties usually give careful consideration as to how many candidates to put forward in the separate Dáil (parliamentary) districts. Transfers are often not along party lines, but rather go towards more prominent local personalities. Election posters for the most prominent candidate of a political party usually list preferred 2nd (and possibly 3rd) preferences for that party.

Electoral stasis

Electoral stasis is a condition that can arise when an electorate cannot change the composition of the legislative chamber, regardless of a swing occurring in a general election. A safe seat in a single-member electoral system is one that is in electoral stasis. Seats under proportional representation systems, such as STV, can also be in electoral stasis. Political party strategists ignore these “safe seats” and allocate resources to other districts.

In an district electing two members, the quota for successful candidacy, 33.34%, is easy for both parties to reach in a two-party-dominated assembly, if their core demographic voter base is larger than the quota. For example, in a district returning three members, electoral stasis occurs when the weaker party has only a quota (about 25%) and one seat, and the more-popular party wins two seats. The situation will only change if the weaker party can rise to about 50 percent of the vote. This change is not likely so the district is seen as likely to produce two safe seats for the larger party.

The fewer the members elected in a district, the more likely that the electorate (and district) will be in electoral stasis. The larger the number of members returned by a district, the greater the chance that a voter that changes their vote can influence the outcome of the election.

In a district with ten members, quota is about 9 percent, and a change from half of quota, not enough to elect anyone, to a number of votes enough to elect one candidate is only 5 percent.

As well, with the use of list PR or STV in a district, even a small change in voting behaviour might produce a change in the representation elected. The ability of a voter to vote directly for a candidate also means that even if a party takes the same number of seats, the members elected might be quite different from election to election. Such election systems also ensure that a party with more votes takes more seats across a city or an area, while first past the post allows a party with just half or two-thirds of the votes cast by that group of voters to take all the seats.

Ballot design

As seen above, voters in an STV election rank candidates on a preferential ballot. The rankings are used as contingency votes (back-up options).

STV systems in use in different countries vary both as to ballot design and as to how many preferences each voter must mark (full preferential voting, optional-preferential voting or semi-optional preferential voting). In the Republic of Ireland and Northern Ireland, voters are permitted to rank as many or as few candidates as they wish. Consequently, voters sometimes, for example, rank only the candidates of a single party, or of their most preferred parties. Some voters, especially if they do not understand the system, "bullet vote", only express a first preference. Allowing voters to rank only as many candidates as they wish grants them greater freedom but can lead to a voter ranking so few candidates that their vote eventually becomes "exhausted". That happens if at a certain point during the count it needs to be transferred but cannot be transferred because there are no markings on the ballot or the marked preferences have all been either elected or eliminated already and none of the remaining candidates were indicated as choices by the voter. It thereby loses an opportunity to influence the result. (In First Past the Post elections, many, sometimes most, votes are disregarded, as there is no opportunity to mark back-up preferences.)

To the extent that voters mark back-up preferences and the back-up preferences are consulted - many are not consulted even if marked - the portion of votes ignored under STV is less than under First Past The Post. Back-up preferences are not consulted if the vote is cast at the start for a candidate who wins in the end as the last seat is filled, or cast for a candidate who is eliminated at the end. They are also not used if they are marked for a candidate who has already been elected or eliminated.

To prevent exhausted ballots, some PR-STV systems oblige voters to give a complete ordering of all of the candidates in an election. (If a voter does not rank all candidates, their ballot may be considered spoilt outright or considered spoiled at the point when the lack of a marked preference affects the count, a loophole used by those who cast a Langer vote).