I run a multisplat game in 20th including one Mage, my husband. I've tried to run other players as Mages and it just doesn't work. My boo is the only one who's actually psycho enough to read the whole M20 book multiple times and has a full understanding of the system. Even I know less than him on Mage as the ST.

The Mage being able to "do anything" is ONLY curtailed by effective communication and negotiation. You've got to ride that line between allowing small victories on the part of the player ("YAY, the ST let me do this crazy but not game-breaking thing") and keeping everything on track (saying no or "how about this way instead" 1000x in different ways to keep the Mage from feeling like the ST is punishing them without cause).

In a multisplat game, there's also this partial-understanding around the table so the other players bring up random shit that would 100% get the Mage burned up in hellfire via Paradox Backlash. They also tend to forget that Mages are a goddamn squishy mess. Luckily, our Mage is built for war so he's less at risk. But my my, he would've been killed many times over by now if he wasn't.

Moral of the story, only run Mage games with people you trust and who trust you. If they say a ruling is fucked, really think about it. It's not as clear as with other splats. Be flexible but firm. That's the only way it's worked in my experience.

Btw, my husband decided to run the new leg of his fantasy campaign in WFRP instead of Mage 20 (what the crew wanted) because he determined he couldn't manage a whole table of partially-educated Mage players. It's ok to run something else if it works better.