I think it's clear that atmuh is referring to this game from a competitive standpoint, while neblix is coming from a pubber point of view.
In a competitive game (or a pub game with decent people), a spy's ability to do much is pretty much based all on luck and timing since most people are aware of what's going on around them. From my experience playing on random pub servers, a spy can actually cause a good amount of chaos, ie: a bunch of silly engineers build all their stuff next to each other; spy saps everything, demo comes in and wipes everything out.
It's like comparing Brawl with all items on with Melee fox only, final destination only, no items, etc.