MySQL features missing in MariaDB Server. Here's an overview from the New York developer's Unconference in February 2018. This is primarily aimed at the developers, to decide what goes into MariaDB 10.4, as opposed to users.
High level comparisons are made between MySQL 5.6/5.7 with of course MySQL 8.0 as well. Here's to ensuring MariaDB Server 10/310.4 has more "Drop-in" compatibility.