I'd caveat the "you don't know the framework well" by first taking a moment to establish that you can meaningfully test the framework. There's a whole whackload of frameworks out there of all different sorts, even very popular ones, that haven't given a single thought to testability (or only thought about it so late in the process it was too late to do anything about it), either of the framework code itself, or the user code using it, and generally this results in a framework that can't be tested with anything lower than integration testing. I consider this a major negative for a framework.

I can't rigorously prove this, but I suspect that also points at having a framework that is going to tend to gratuitously break your code frequently if you try to upgrade, because they'll have no tests for telling if they have. And neither do you.