It only affects it as much as it takes your exotic slot away but their are enough legendary heavies in the game now to use a exotic primary and I think that can be Witherhoard or fighting Lion, Mainly there are much stronger exotic secondaries to use over FL.
Idk about that. If you look on the exotic secondaries, they are more PvE focused. Unless you use Jotun, fighting lion can be better than those. Even then, there are a lot more stronger primaries like Ace, last word, thorn, sturm, crimson, chaperone, malfeasance. It has less competition as a secondary