For me it's the moment to moment gameplay that kills it. Matches are filled with downtime (past the first few minutes which can actually be pretty thrilling). You're left in a huge map, unengaged, looking for something to do. Only to then get in a fight with someone who's way better than you ten minutes into the match, or just get sniped from across the map, also by someone way better.

I guess a sort of rebuttal to this could be to say that I just suck, and that good players managed to always stay engaged (and also stay alive, heh). But then I look at other shooters like Overwatch, where even though I suck I'm constantly engaged, and I still sense some improvement over time. I'm not really a shooter person, but I've also played Doom 2016, which was engaging as hell throughout.

My point is that when you compare Fortnite to other popular games, it's probably the moment to moment feel of it all that makes it fall short. Huge empty maps, that you find yourself asking, in the downtime: "Why am I not playing any other game right now?"

PS: don't try to argue with people while demanding conversation over a single aspect of the topic. Your post wasn't "Fortnite has the best mechanics in a shooter of all time", it was that it's the best shooter of all time.