FM has a major advantage over AM that can be summed up in one word --
limiting.
You can get perfectly flat response without having a perfectly flat IF strip.

I am not sure you know what you mean here.
The FM IF limiting is done to remove all AM from the 10.7 mHz IF signal.
The only information is the frequency modulation.
Foster Seeley FM discriminators are sensitive to AM which causes distortion.
So makers use limiters prior to the FSD.
Ratio Detectors are less prone, but slightly higher thd.

I did know what I was talking about, and you gave excellent confirming examples.