No one said anything about limiting Shiranui. What they want is limiting the grass, as it gives too much advantage for a first turn. Lightsworn, infernoid and such would be broken as well with this card on their first turn
Like any deck, if you get a lucky play you'll win, and if you don't, you'll lose. I've beaten plenty of grass decks with Cyber Dragons (and lost to plenty of them too). I haven't felt like they were too OP compared to other archetypes (looking at you, Darklords).
No deck will ever be as abused as when Sylvians where originally released. Self-milling your entire deck to nuke the few cards your opponent had and proceed to fill your entire board to sweep for game.