Drupal - What complications are there if I switch Mysql to MariaDB? Any Drush issues?

As you say, Maria DB is a drop-in, completely transparent replacement for MySQL. Its releases coincide with the same major/minor version of MySQL so it's pretty much always in tandem as far as features are concerned. It reads the standard MySQL binary data files, uses the standard systen my.cnf, and even has a drop-in replacement for InnoDB.

The idea is that as far as your application is concerned it thinks it's connecting to a MySQL server. It uses MySQL drivers, issues full MySQL statements, and receives responses exactly as the MySQL server would send. Your app(s) won't know the difference.

I've been using Maria for a while now for Drupal sites (also using Drush extensively) and haven't had a single problem to date. If you're running *nix upgrading is just a two minute job.

Just wanted to chime in on this (albeit months late)... Have set up many Drupal sites in the past, decided to do things "better" this time and had MariaDB installed.

Everything works wonderfully (faster, cleaner, etc) with Drupal 7 EXCEPT for backup/restore :/ Always have to go directly into the db (whether through PHPMyAdmin, Heidi, or command line) and copy/export all of the tables.

Other than that, which there could be a number of reasons for happening, I highly recommend MariaDB. Less server resources used, D7 is much faster, etc etc.