She sneaks no worse or better than any other Housecarl, and she gets in the line of fire in melee combat because that's what all melee combat followers do. She's also the easiest to obtain and she's centrally located at Whiterun which is why I use her all the time. The fact of the matter is that, like all Housecarls, she is one of the best melee combat/archery followers in the game. Her AI is no different than any other Housecarl when it comes to combat and sneaking which in turn is no different than most melee NPC followers, but because players find her sarcasm about carrying their burdens annoying they imbue all sorts of other non-existent unique negative qualities about her as a follower.

I find it amusing how players lose all objectivity about her due to one sarcastic line in her dialogue.