> bullshit reasons

Depends on the reason. "Bullshit" is in the eye of the beholder.

I don't think that they reject for branding issues, in the TestFlight review (like they do, for the App Store -I have had my app rejected, because it had the word "finder" in its name). For example, if you change the color of the Sign in with Apple button, it will get rejected (in the App Store review -I don't know if they would reject it in the TestFlight review).

They do, however, look for things like private API access, crashes, and dodgy behavior. Even if a code path is TODO, it shouldn't crash. I just fill those with NOP functions.