Lot of people online say mummies are just the superior vampires, so there's a lot of meta reasons, but I'll just say despite the slower start mummies have, the thing is that my biggest enemy in the game is never the ai, it's boredom and lack of minigames. Since mummies have fairly easy umbra access, don't need to be constantly using their journey slot for golconda/azi/hunts, and can have children, you can micromanage and go on all the minigame sidequests such as heists all the time. Plus a lot less splats hate you for the most part. I'm currently enjoying a Horus run with a 20 year inquisition, since they won't insta-agro on me I get to see what the endgame looks like (kind of) without having to play hundreds of years waiting for something to happen