The attraction of mummies is supposed to be the lack of hunger. The issue with that is that none of the hunger costs in DCSS are (or were) significant except for 1. preventing you from farming indefinitely, and 2. old Elyvilon. Mummies couldn't worship Elyvilon in the first place without exploiting a bug. So the only real advantage it confers is that you can farm.
In 0.5 and earlier, Mu could safely farm D:1 (and all other levels) indefinitely, because monsters never stopped respawning and the OOD timer was a joke at the time. In theory, this was quite strong, but you had to be able to clear D:1 to be able to farm it, and D:1 was actually potentially hard back then. So Mu wasn't an attractive streak race even if you were willing to spend hours farming. This is not theoretical, by the way - you can find quite a few old games where players farmed with mummies for hundreds of thousands of turns, such as
this one.
In 0.6 (I think?) the OOD timer was made a lot harsher, and grinding Dungeon was not a viable strategy since you had to be able to deal with the centaur warriors etc. that would start spawning. However, you could grind Lair for 20,000 turns per level fairly easily, since Lair didn't really have any OOD monsters in it, and monster spawns stopped after 20,000 turns. Eventually you could do the same with other branches too if you wanted, and were powerful enough to deal with the most dangerous monsters there. But doing it starting on D:1 was nigh impossible - you can look at the logs for the mummyrobin account (a round-robin account dedicated to the practice) to see how hard it is.
And now recently monster spawns have been removed altogether in most branches, so mummies (and vampires) have a bigger advantage again; they won't get much extra xp from waiting for thousands of turns, but they sure can get a lot of extra safety. No need to explore into dangerous terrain, you can stand on good terrain and wait for all the monsters to wander there. Got some monsters on the other side of a staircase? Just wait 10,000 turns and they'll disappear. This is a pretty powerful ability, though obviously not enough to make up for losing potions. It's also no fun at all and blatantly violates DCSS's design goal of preventing grinding.