When is noise reduction 'too much'? Knowing when to push for ADR.

Hey everyone, I'm a 2nd AD dealing with a lot of on-set audio lately and trying to get a better handle on post-production audio decisions. We're cutting a short film right now shot on an ALEXA Mini, and some of the dialogue captured with our MKH 416s has a bit more ambient hiss than we'd like. Our sound designer tried some noise reduction, which definitely helped, but I'm worried it's starting to sound a little 'phase-y' or unnatural. The guide I'm referencing talks about 'over-processing dialogue' as a common mistake. My question is, how do you really know when you've hit that point? Is there a good practical benchmark? At what stage do you, or your team, decide to just bite the bullet and recommend ADR instead of pushing the noise reduction further?