As far as I am aware there is no good reason to not use MariaDB instead of MySQL. The creator of MariaDB, Monty Widenius, was on a episode of the changelog podcast if you want to hear some background as to why.
I've been to a Monty talk about this subject, and let me say it was interesting...
Monty starts by saying that he does not acknowledge MySQL 5.6's existence since some new features are an abomination (usually citing GTIDs).
He then compares MySQL 5.5 to MariaDB (of which MariaDB is based on MySQL 5.5; so it contains the superset of features).
MySQL 5.5 was released in 2010. Support for it will end at the end of this year.
Development has diverged a lot since 2010. For a more balanced comparison, I recommend this one: https://www.percona.com/blog/2017/11/02/mysql-vs-mariadb-rea...
Large companies have come to a different conclusion, at least so far. Among companies with very large database fleets, there's much greater usage of MySQL and Percona Server than MariaDB.
Granted, Linux distributions have leaned more towards shipping MariaDB by default instead of MySQL. This makes sense given that Oracle is, well, Oracle.
Just a personal experience.