Horses for courses
Gosh so much hate in the comments.
Clearly some people have found MySQL/MariaDB does not work for their use-cases but its popularity also suggests its quite good for others.
My own experience is that we have been pretty happy using it for a long time now. (Our current database seems to have first started Dec 2006 but its had various version/hosting updates)
Interesting to hear that people think PostgreSQL is faster at joins. I use joins a lot for aggregation queries that are hugely faster in SQL than trying to do the equivalent operation at the application layer. I don't see us putting in the effort to migrate though.
We started before foreign keys worked properly but it was never really much of an issue because the majority of our data is fairly static once created, records that do experience lots of updates never update their reference fields after their initial creation and its very rare for us to delete records.
If I have any niggle its that table metadata operations are a bit slow. Nothing thats a problem in production but its noticeable in unit tests where each test tends to re-create all the tables it uses.