Sadly, I've only heard anecdotal evidence from young teachers I know, and maybe some reading I've done in the past, so I don't have any sources on hand to point

Sadly, I've only heard anecdotal evidence from young teachers I know, and maybe some reading I've done in the past, so I don't have any sources on hand to point to, and maybe it's isolated rather than endemic. You're right that it's hard to quantify whether someone is a good teacher or not, but I don't think it's impossible to judge. It's probably impossible to make it a simple succinct rubric, though. Maybe it's a challenge for a universal function approximator like a neural net :-)