Imagine a world of magic. This magic can do anything you want it to, as long as physics agrees (totally useless addendum: physics is actually a very pendantic guy, but rumour has it his mistresses have been seen flying).
Meaning, energy is to be conserved. A spell can lay dormant for centuries, but once activated it can only consume the amount of energy put into it when casted. Which is why magicians are very good cooks, they have to get fat fast before casting a powerful spell and then consume their fat all at once to put the energy into the spell (others just kill animals, but that's frowned upon).
Now skeletons are easy to use as tomb guards (scary, bones are easy to get), but how much energy would be needed (approx.) to have a skeleton actually be able to fight? They don't have muscles etc. so keeping all those bones aligned would be consuming some energy already. Is it feasible to cast the skeleton spell and the remains of an army within a few days/weeks/months without having big energy sources available (aka food, maybe a companion), or would those skeletons rather need more energy?
"Careful, there are skeletons there, they'll chop you into parts!" "I know, just watch." The hero charges into the dungeon activating all trap spells, turns around, sprints out of the dungeon, releases the stick holding the boulder next to the door, waits two days, removes the boulder and walks into the dungeon because the skeletons had only enough energy to hit the boulder once or twice and are now just bones lying on the ground.
I'd like to evade that scenario.