STV has its uses but larger multi rep constituencies 1 breaks the relationship between voter and politician and 2 allows extremist parties to act as king makers see the DUP in the UK and much more so in Europe
The DUP as king makers in the UK has nothing to do with STV; in fact it is an example of king makers with no STV. There are no STV or multi rep constituencies in British general elections. (In Northern Ireland, STV is used for local elections, but that's something else.)
Large Multi representative constituencies do even with STV eg UKIP getting MEP's but Not MP's