The biggest problem with contaminated runways is that when you are in dynamic situation i.e it’s starting to snow or raining heavily it can be hard to know when the runway is contaminated and what the braking actions are
Exactly that. It's not an exact science. And we do divert when the numbers are really down, but we can't divert for every reported reduction of braking action. So where do we draw the line in the sand? Pilot discretion. However, one tool you have at your disposal is is to drop down a bit below your normal descent path and touchdown earlier than what the performance app assumes - i.e. " duck under". Done with caution, not a big deal on the 737. I won't speak for longer bodies. However, that discussion was equivalent to opening a can of worms.