Shamans can tank all 5 mans just fine and even Blackrock Spire (10 man). Shaman tanks will always take more damage than any other tank class because they can’t wear plate or get a big bear armor modifier.
Shaman threat is just fine between rock biter and earth shock and stormstrike lightning shield, especially at lower levels. Shamans have the same issue that Paladins have for tanking (and this is the #1 reason you don’t see Paladin tanks in raids), the blue rage bar. You have a finite amount of threat you can pump out over the course of a fight because of this. In 5 mans though, this just means you’ll probably just be taking more mana breaks or drinking more mana pots than the healer which won’t be an issue for most groups if you handle everything else well.
You’ll also have to deal with the stigma that “shamans can’t tank” which will keep you from being invited to some groups. Nothing you can do to change that other than showing people you can actually do it, so they’ll invite you again next time.
Just remember that you will probably never really be able to tank raids as a shaman other than the occasional trash mob.
As a side note, you have one thing a warrior never will, a ranged pull that will give you a good bit of initial aoe aggro with chain lightning.