In general, for the size populations we are talking about it's probably unreasonable to expect to walk into a camp dressed as one of them... with a small enough population they probably already know each other by site pretty well. But who knows?
I guess it depends on the uniform. If that uniform constitutes a full suit of armor and a helmet, it's pretty hard to see through that kind of disguise save for the player looting gear that doesn't fit them right. Otherwise, I'd say that a disguise would work reasonably well at night. If you're infiltrating a clan or something that uses a distinct emblem in the uniform during the night, I'd say it's pretty unlikely that they'd be able to get a good enough look at your face to care, and I doubt they'd go out of their way to try and make out your face in the darkness of night after seeing their emblem on your uniform. During the day, though, sure, they might get a little suspicious... or they might second guess themselves. If they're anything like me, then they're face blind and are just proud of the fact that they can tell someone's face apart from a loaf of bread. :p
Anywho, as long as stealth is actually realistic unlike the handicapped or punishing stealth that most games are graced with, I'm happy. There's nothing less immersive than NPCs with the memory of a goldfish.