Mummies are another one of the splats that just don't play very well with others, like Vampires and Prometheans. To start with, they're obscenely powerful but also effectively insane at 'character creation', and they 'burn out' rather fast. It's hard to have a group where the Mummy is not the most plot-centric character simply because if they are not the most plot-centric, they would be off dealing with their own problems while they can rather than bumming around with other supers.
A circle of mummies working together is very functional, since their goals will usually align or overlap (along with their enemies), and a solo Mummy can make for an extremely dangerous antagonist - both as a behind-the-scenes manipulator and during the inevitable combat confrontation. They're just problematic in a cross-splat teamup.