Pistols are honestly the worst suited for CQB environments. Pistol accuracy is perhaps the biggest understatement I've made. You MUST be able to place the shots as accurately as possible, and that assumes you're a fairly good shot. Head shots are probably your best and only bet, because you won't be able to rely on your dumb luck to take someone down shooting out their legs. Headshots are tough, and they're even more tough in an environment where latency is always a consideration.
Pistols have no zoom whatsoever.
Most SMGs at least have some degree of zooming capability. Most engagements occur over a marked distance, where accuracy is not paramount. These pistols have a lower magazine capacity than any other weapon, except a sniper rifle. I've added this caveat at the end because I would hate for anyone to think that I'm actually advocating the use of the pistol as your only weapon. I am, however, advocating the use of pistols for situations other than in CQB situations in which your primary is empty.