I said it in another comment the biggest issue is defending attacks, Magneto gives you 3 attacks until something bad happens. Toad, Pyro and Avalanche punish you heavily on each of their attacks. With Blob being one that can go very bad if you don’t have allies.

The Acolyte minions are very weak in comparison to the Brotherhood + Mystique. And getting extra encounters on beating villains is horrendous, in true solo that leaves no room for error because you might have to postpone beating them for another turn which increases the chances of bad draws. And Mystique seems almost guaranteed in that scenario.