Under proportional representation, government would be more stable and better represent different groups. At present, a small change in electoral results can lead to a 100% shift in power. This inherently leads to many inefficiencies as one government starts something, then the next tears it down and starts something new.
While it’s fine to try new things and abandon plans that prove dumb (or at least not worth the money), the more stable electoral results would tend to provide more stable policy, which would facilitate higher quality and better implementation of policy.
The main problem with proportional representation is that it could be “too stable”, and so any proportional representation elements would need to be designed in a way that makes it easy for a) party members to kick the bums out of their own party at times, and b) for voters to penalize parties whose members prove unable to do so.
In short, being “too stable”, additional (democratically empowered) anti-corruption measures would be needed to reassure about unseen insiders and party lists, etc. Having “lists” at a relatively regional level, not national level, could address that.