there are decks and decks.
Mind gate can be good vs AI3 and sometimes in pvp. Not that great in arena when you try to farm.
It's main weakness is the requirement of a big quanta pool early on and sustainable. Can be great realy on against a rainbow which uses cheap creatures. It will work bad against any deck that uses high cost creatures, against decks that plays only pendulums (usually you'll require a specific quanta that you will not be able to steal from enemy pack), against PC decks, especially in arena where double draw will amass tons of PC rather fast.
Not to say that sometimes I don't understand how it works at all, I see oponent with no double draw playing a pillar, but I previously just got a creature for which I have no quanta. Why didn't I got the pillar ?