Thats not true. I'm pretty sure you can still exile it or detain it. Maybe not with a spell but creature abilities should still detain or exile it. And it can still only block one creature at a time. Swarm, white weenie, minotaurs. anything that can be played fast and numerously can win against it. Besides mana mastery decks take too long to get going and it's much easier and much more fun just to use mono or duo colored decks.
Ahh I totally forgot about door to nothingness. I'm also looking for an artifact card I can't remember the name but ability is" you cant lose and opponents can't win" but of course I can't find one anywhere