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> I do think it's important to have unit tests when the unit's behavior is complicated. Where I start to get worried is when there are unit tests being written against classes that have very little behavior that doesn't involve interaction with some other module.

I think this is precisely where unit test suits start to have problems. Good, flexible unit testing requires a lot of judgement about what will be useful to test and what will be too much of a burden in the future. Unfortunately judgement is hard to acquire and even more difficult to teach, and a lot of teams want to create and enforce over-dogmatic testing "standards." When unit testing, you have to balance:

1. What testing do I need to have confidence my code is working?

2. What testing do I need to catch likely regressions?

3. What kinds of tests will just get in my way in the future or are literally useless [1]?

[1] E.g. unit tests that essentially only test core language functionality, once you take out all the mocks.