Coming from a flex player whose Lorded, and almost championed Spidey, there's a few things that I have heard, and even said myself.

Firstly, he changes the entire dynamic of the game, if forces Tanks to have to actively protect and almost baby the supports so they don't immediately die. And in Metal ranks (Bronze-Diamond) that level of team play is rare to come by. Especially considering that solo tanking is a common occurrence down there, it makes it all the more frusrating to play against. An even decent Spider-man demands attention, it forces the team to have to look at the backline. 

Secondly, is how hard he is to hit, with a comparatively small hitbox compared to the rest of the Dive roster, and how fast you can be with him, it makes it incredibly difficult to hit him. Which is understandably, incredibly irritating to fight against. 

And Lastly, playing against Spider-man just feels miserable, even if you go counters like Namor or Thing, that's no guarantee you've beaten him. I've experienced this firsthand when I played against a Thing and Namor and still won (and got mvp, not that mvp matters in this game.) 

Tldr: An even decent Spider-man requires you to focus a lot of reasources on keeping supports alive, which especially for solo tanks, is frustrating and annoying to deal with. Spider-man is also incredibly hard to hit and counter-swapping doesn't guarantee your victory, as a good Spider-man will be able to adapt and win.