The point is that you use a tool so that you don't have to maintain a given subsystem. If you don't have to maintain it, you don't have to test it. So, you can test a single RDBMS (mysql or pg, for example) and be reasonably certain that it will all work in others.
My criteria for good software:
- Does it work?
- Can someone else come in, make a change, and be reasonably certain no bugs were introduced?