Except when you test that something is *not* an instance of something else and the test wrongly passes. Then you'll be scratching your head trying to fix a bug already in production.
I've never written such a test assertion in my life. I'll be fine with Jest
That's no the issue, the issue is if the code you can't access or control uses `instanceof`. If the only problem was you needed to avoid `instanceof` in tests, this wouldn't matter at all.