Terrible choices: MySQL
blog.ionelmc.ro
blog.ionelmc.ro
For example I code using Drupal alot and to my knowledge the PDO settings for the _supported_ database engines attempt to make the Database layer API provided by Drupal as consistent across all underlying DBMSs for developers.
If I were to add a driver for say FileMaker Pro to drupal, it'd be up to me to assure the underlying FileMaker Pro DBMS settings and semantics were configured similarly to the default supported drivers in Drupal.