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.