Curses screw up your luck. That hit to the head that you desperately need that just happens to be a very easy, square hit? You have a good chance of missing when cursed. Likewise, you (or one of your companions) can be cursed by disturbing a mummy; that is, if you wake it up and it's hostile* it has a chance to curse you (or a companion). IIRC, the mummy needs line of sight (and only line of sight) to curse someone.
*Mummies will not be hostile (unless you attack them) as a necromancer. Some mummies can even be recruited. Another interesting thing about mummies is, once disturbed, they may leave their tombs and travel the world. You can even get them to show up in fort mode.
Also, the loot generally isn't worth it. I really only go into tombs if I'm a necromancer in need of a quick army.