I understand this very much, but I have one big difference in experience: Once your coverage reaches about 60%, tests expose crashes simply by running the code.

I understand this very much, but I have one big difference in experience: Once your coverage reaches about 60%, tests expose crashes simply by running the code. Even if there are no assertions. I've tamed a few untested code bases by writing huge integration test-ish unit tests, and comparing the output with pre-recorded answers. After that, you start adding real unit tests for fresh code, knowing the existing stuff is quite safe.