While it is indeed often faster, it isn't drop-in. MySQL and MariaDB have diverged over the years, and each has some interesting features that the other lacks.
Wow, I hadn't realized that MySQL and MariaDB diverged so much! In the last year I've started seeing some prominent applications like Apache Superset and Apache AirFlow claiming they don't support—or even test on—MariaDB at all.
I wrote a summary of the DDL / table design differences between MySQL and MariaDB, and that topic alone is fairly long: https://www.skeema.io/blog/2023/05/10/mysql-vs-mariadb-schem...
Another area with major differences is replication, especially when moving beyond basic async topologies.