Small worlds under ground and mummies
The fact that there are 10 is the good side.

This is similar to other races from the base Small World and its expansions. For example, ratmen get a larger number of tokens with no downside. Kobolds (never leave a kobold token alone), barbarians (no redeployment) and pixies (only 1 stays on the board in each region) all have even larger token numbers but have some kind of downside that let them play differently. Kobolds are good at spreading fast but only hold a normal amount of territory. Barbarians spread fast over lots of territory but can't defend it strategically. Pixies are like barbarians on steroids; even more tokens but even less ability to defend.

Mummies are similar; they get more tokens, allowing them to hold lots of territory, but they have a downside that makes them spread out more slowly. This can go well with powers that reward them for holding particular regions (the territory powers), or that want them to stick together (frightened). It also does well with certain powers, such as Tomb, by making the race much more resilient in decline. If you get a power or other ability that makes attacking easier (such as the rabbit sword), they can spread more quickly and hold more total territory.

Mummies are good when your goal is to build up a strong board presence, where consistency is more important than speed. As such, you generally don't want them as your first race unless they have a strong power combo. They'll do well when your previous declined race is still going strong, taking time to spread wide into their own large decline. Reborn is a great combo for your previous race, helping mummies spread quickly and then minimising the loss of previous pieces when they're ready to decline.

Additionally, as you mention, if the mummies aren't a great pick for the current game situation, they'll accumulate money as people skip them, making them more valuable later on.