Somewhat naive too to be honest. As someone that had just plain old ruby circa 1.8.5ish code (not rails) that I ported to 1.9 a while back, even with tests there were differences in behavior that I guarantee you probably weren't thinking of writing a unit test for.

Unit tests are nice, but they don't make things braindead easy. Just make it a bit easier to narrow down where something might be broken/different.