They are pretty tough, but hopefully, at the level where you are facing them, your party has a leader or other character who can throw out some extra saves, or some bonuses to saves. Assuming you do the poison attack as a secondary vs. Fortitude, the drow has to hit with the primary, hit with the secondary, and then you have to fail two saves to be forced into unconsciousness.
A series of bad rolls can mean that your party all gets knocked out, of course, but I would hope that a party of that level had some magic items or abilities to help with saves, and I would hope that any party facing drow would take some precautions (anything that helped resist poison, etc).
Of course, my group, knowing that they were facing the drow, and having fought the drow before, blithely ignored the opportunity to purchase, decant, or enchant anything that helped vs. poison. I think they are just going to count on the Dwarf Barbarian to deal with them. "You get +5 on your poison saves and you have the best Fort defense anyway. Go rush those snipers, Mr. Pincushion!".