> "all the magic"
The "magic" factor in RoR is much less than, say, iOS development in XCode. And RoR has the benefit of being text-based, so whatever functionality you're looking for, you can be sure it's in a text file somewhere.
> made even worse by dynamic typing
This is a feature Ruby shares with the #1 and #2 most used programming languages.
> how bad a time you'll have if the test suite's absent, or stale & neglected
I agree that having no tests or bad tests guarantees a bad time, but I'd say it's pretty standard for even the bad codebases to be covered pretty consistently with unit tests. The Rails community has, by and large, accepted unit tests as part of our shared culture, so if your developers adhere to Rails convention, they will maintain decent test overage. That's a strength of RoR, not a weakness.