There are non-abstractable differences once you no longer have a toy project, e.g. locking and isolation semantics.
There might be some valid reason for making your applications talk to different DBMS, but you have a tough slog ahead no matter how you do it.
Yeah, sure, go tell that to your manager who promised multi vendor compat to your clients.
Problems with the ORM nowadays always stem from developers who are ignorant of SQL. Those people should not be working for you anyway.
Okay, I'll tell him that 5x the systems means more work, possible bugs, etc. Seems logical.
I'll tell him the same thing if he promises Windows, Mac, Linux, Solaris, and Plan 9 compatibility.
I'm not saying you shouldn't be able to use the same application with several databases; I'm saying you should avoid it.