Getting approved by everyone/enough people would be the biggest obstacle. there is a checklist as well, but I am assuming that could be worked through. also you are generally not "allowed" in if you have ongoing conflicts. sometimes that is broadly defined, other times its very narrow.
Draw backs would be the internal division. as a member they would have some veto powers. and there is some debate about how Article 5 would play out between two NATO members. There was some debate years ago about nations going to war inside of NATO and how the rest would/should react. always tough to prove who actually shot first, and what happens when the victim of an attack is actually the first to fire.