Well, here's a case in point. The Australian state of Tasmania uses STV for electing its House of Assembly (equivalent to the House of Commons). The final count is now drawing to a close and in the seat of Lyons, a long-serving Labor MP, David Lllewellyn, has been unseated by Rebecca White, who also stood for Labor. It seems as if Ms White even campaigned against her fellow Labor candidates.
This is yet another reason why neither Labour nor the Conservatives will ever embrace STV with any enthusiasm.