My own personal anecdotal evidence is that mysql is fine, until you have complicated queries or a lot of data. And then it suddenly sucks. We had a huge problem

My own personal anecdotal evidence is that mysql is fine, until you have complicated queries or a lot of data. And then it suddenly sucks. We had a huge problem where the query planner refused to use the indexes we had unless we manually put index hints in the query (yuck). Rather than actually doing that, we migrated all our data to postgresql. The postgres query planner works much better than the mysql one. Since then we've started using some of the postgres specific features, like windowing, which have made complex queries that we didn't think were going to be possible not only doable, but easy and fast (running). Postgres has some different philosophy about certain things in the devops side of things, but if you don't try to force it to be like mysql it'll be fine. At this point I wouldn't ever use mysql again (for new projects where I have the choice).