It's dangerous!
I've done it (and will certainly do it again). As DM, you have to realize that the fight is going to be very dangerous for the PCs. If they have had previous encounters, or expect to have future encounters on the same day, they will probably be holding back to save resources (or be lacking resources).
Also, surprise, terrain, knowledge of the foe, and other strategic issues become very important. If an NPC party with good knowledge of the PCs ambushes the party, the party is probably going to lose.
Lastly, watch out for designing the NPCs to attack the PC's faults. I've done this without realizing it in the past (in superhero games, this is really easy to do).