We migrated a high-traffic, complex site with a huge amount of data to MariaDB about 5 years ago as well, and it's been solid so far. We use Galera as well, which has performed and scaled really admirably.
I have a lot of stored procedures and functions that were converted from Oracle to MySQL, and they also work just fine in MariaDB.
I guess any actual incompatibility is in fringe cutting edge features, which is where both databases have diverged.
Anecdata: I've been using mariadb as a drop in mysql replacement for over 5 years in production, and it's been working seamlessly.
I'm no DB expert, and perhaps my use case isn't complex enough or written in a highly mysql dependent way - but I felt like someone should chime in since there's not much positivity toward mariadb in this thread so far. I'd be interested to hear from some other people with actual mariadb experience.