Volibear doesn't need to b after his first clear though. Voli might go through 2 or 3 potions on first clear, meaning he can afford to do stuff after getting second camp. Twitch risks giving both buffs on a gank because junglers who cleared it faster and safer can effectivelt counter gank him, and his natural squish means even a quick concentrated burst + ignite from one person could very well end him.
Plus voli has flip, aoe slow, and super strong early execute. Much better ganks than twitch invis with casket.