I'm assuming people have done it, but if I did, would I lose anything valuable from NCR quests or anything if I became vilified by them?

Just about everybody and their dog has done it. You lose all NCR/Crimson Caravan quests. Not sure if you can just disguise yourself and do them.

they'll shoot you on sight, along with any other security if they see it happening. you pretty much won't be able to go anywhere without getting shot at. and most of the quests are NCR or require that they aren't hostile to you. some companions won't join you.

and i did finish the game siding with the Legion once, but the ending was very unsatisfying. I was hoping they'd blow up NV or something, but instead they just enslaved them to conquer more.

Yes I beat it. I liked it better than the other endings although I have yet to do the NCR ending. It felt good for once to see the NCR soldiers getting owned. Plus it was a good battle a hoover dam. Anyways yes you will be vilified by helping them and it will be hard to get into alot of cities. Unless you have NCR armor/clothing. And you can still do some NCR side quests like the "Lucky Old Sun" but you need to be disguised with their armor.

If only you could join the Brotherhood of Steel.

You can.

The Brotherhood is simply too weak to actually content with Caesar/House/NCR.

You can join them. Also, if you go with NCR or Yes Man you can get them to fight with you at Hoover Dam.

I've done it but i made the minor mistake of siding with Legion too early. They kept sending Rangers after me before i could get my hands on AP ammo. It didn't help that everytime they showed up to warn me, i would start blasting on them with my wimpy varmit rifle and rusty 9mm. LOL!