MM is limited in every aspect. It has lower range, every spell has a different range, easily interrupted (ridiculously so), armour penalties and self interrupted by turning.MM is really powerful so it should have limitations. It's MA that is too strong atm, which makes MM seem weaker in comparison, but to get back on topic, with my suggestion an MM with PSY could then be more viable.
Imo the damage and armour bypass are not enough of a pay off for the limitations. I'd be happy to lose some armour penetration if they reworked the interrupts into delays instead. Armour penetration is useless if you never get to cast a spell.