> even when the real reason is something like "meh I'm not sure the candidate quite clears our bar".

That's the problem, though - you're trying to make a hiring decision on hopelessly incomplete information, and so it will come down in large part to gut feeling. And the moment you admit to using some non-quantifiable feeling as a basis for not hiring someone, you open yourself up to claims of some form of discrimination.

It's all around safest to just stay quiet.