They can be a bit off putting to new players. You know the scene, new guy turns up at totally unfamiliar site wearing a hoody and jeans and gets handed an adequate but not very cool full face mask and a battered JG G36, then sees five people all dressed up like they are about to do a special forces halo jump out of a C130 into occupied enemy territory, and they are not socialising with other players, but only talking amongst themselves, testing their comms and gassing up their fancy sidearms. Having said that, if new guy sticks at it, he'll probably end up like those guys in a years' time, but I usually try to make a point of talking to guys who look like they are new to it, to at least make them feel welcome.
Another down side to it is that such teams are often a shoe-in for victory when many other players are walk on for the day, impromptu scratch teams, formed on the spot, some of whom do not know the layout, which means they probably won't win, even though they may very well actually be better or smarter players. That is the nature of the beast of course for anyone new to any endeavour, but I've found myself teaming up with those kind of site-regular 'pro' teams on occasion, and then had guys shouting commands to me like they were some kind of veteran Sergeant Barnes and I was the FNG in some bad war movie. In such situations, I've gone with it for the benefit of the fight at the time, but whilst doing so was often thinking: 'what a fecking tool'. Especially when a lot of the time it seemed like they weren't actually issuing very sensible orders either and didn't have very sound tactics.
I'm not averse to people 'getting into it', but I do sometimes think the odd player really does take it a bit too seriously, and in any case it's generally not the best way toward teamwork either. Had that once with a flying instructor, he was barking orders at me from the rear seat of the aircraft like some low budget version of James Robertson Justice, as though I was some new pilot officer on my second lesson in a Tiger Moth in 1938. There's me sat there in the P1 seat of the aeroplane thinking, 'hang on a minute pal, I'm paying for this sh*t and doing it for my own personal enjoyment, we're not preparing to stem the Nazi hordes, I'm getting a PPL, d'you wanna chill out a bit?' In fairness though, that experience did actually teach me a lesson that I've carried forward into my job (as a trainer), even if the lesson was, don't train people like that f*cking idiot was doing, because it's often not the best way to connect with people.
So yeah, if you're into it, teams can be good, and most of them are, but some of them come across like a bunch of wannabe cock ends, especially the ones who won't take hits off new players, since they think it's somehow beneath them (and yes, I've seen that on more than one occasion).