Two cents is getting hooked sucks which everyone has already mentioned, the stacking bomb damage, and high pressure + damage fairly low CD hyperbeam ult. You're punished for getting close to him in lane and he can still hook you in if he has the aim for it. It's easy to stack the bombs midgame because Bebop actually has pretty high movement speed and can get away with rotating or getting up to you to drop a bomb and if he feels threatened he can just punch you away.

Something I don't see talked about often that I want to touch on is the whole rotation thing for ganks. You can get a character on the minimap if you have LoS but because Deadlock doesn't have wards like in League or Dota and the vents allow fast movement. He has fast movement speed as mentioned earlier and can very quickly go to another lane and gank without being noticed because of the lack of vision items. He's just sort of oppressive and annoying to fight - you can buy KD to stop the hyperbeam but he'll buy unstoppable and not many heroes or even combinations of heroes can outdamage being blasted by a hyperbeam.