"FC" is Federal Cartridge, as you probably know. I haven't heard that they are not to be used in AR's with "upper end loads". Think about it, if the factory loads, which are almost always upper end loads, worked fine and didn't cause any catastrophes, why would any reloads using the same cartridges not work just as well?
I can't imagine any modern cartridge not being strong enough at the web for any reasonable load you want to use.
What is the source of the advice you got?