I’m not very well-versed on all this but it seems
First Past The Post
Benefits the two parties in a two-party duopoly system like that of the US. Boom or bust, black or white. When the party in power pisses you off you vote their competitor even if holding your nose.
Seems like there must be a better way, maybe just not as good for those who prefer shooting fish in a barrel
That’s basically the main downside I see to PR, finding out your local MP is from the monster raving loony party would be rather annoying. Saying that, I doubt he could do a worse job than the useless tory bint I currently have ¯\_(ツ)_/¯