- A shift from first-past-the-post to PR shifts party competition from being within parties to being between parties. It is not clear that this is good.
- Under PR you get more parties. This includes far right parties. There is a significant advantage in future electoral competition once you've become an established party.
- Coalitions have the problem of it being hard for voters to hold specific parties or leaders to account for policies.
- Coalitions create a free rider problem because the reputation of the coalition government is a public good.
- The median voter theorem no longer holds. If voters have extremist tendencies there is no competitive pressure not to serve them
- Italian and French politics look not good. Having non FPTP seems plausible to have played a part in this in that it allowed Front National to gain a significant foothold in France while no equivalent far right party has sprung up in the UK. The Northern League is not fascist but is hard right and is currently the largest party in the Italian Parliament. The Brothers of Italy are literally neo-fascists and have been part of Italian coalition government and plausibly could be again.
- Big tent Social democrats competing with Christian Democrats seemed to work fine.
My claim isn't that first past the post is clearly better than other electoral systems, but it's not clearly much worse and so I wouldn't expect it to pass the very high bar of being an EA cause area.
An important point note is that these arguments apply much less strongly to the US because of the much weaker parties.
I may or may not write a more detailed account of this, but given my record of trying and failing to write good forum posts, this may the best I'll do.