I can see someone who is very particular about their way being the right way having issues with it. I’m very much the kind of person who believes that if I can’t write a failing test, I don’t have a very serious case. A lot of devs aren’t like that.

Sometimes you’re unable to write a failing test because the code is such that you can’t reliably reason about it, and hence have a hard time finding the cases where it will do the wrong thing. Being able to reason about code in that way is as important as for the code to be testable.