I did this very thing last night. I mixed on headphones because it was late and my son was asleep in the next room, so I whacked on the cans to do a spot of riffage, listened to it when I got up this morning and it's gotta be the most unbalanced mix I've ever done. Headphones lie to your ears so bad, creating an illusion of space and volume that just isn't there in speakers
Personally I think if you manage the balance between monitor depth and headphone depth, it is possible to make good headphone mixes (took me a long time, admittedly).
IMO, headphones allow the better fine tuning than a decent treated room.
Also it seems to me, that if you listen for an hour or longer (without break) to a mix on headphones, the perception of rooms somehow increases .
When I first experienced this, it somehow felt as if i was listening from inside the mix. (No drugs involved BTW).