Personally I think all of our companions will have a reason to betray us, one that will come up by the end of the act, with our actions up to that point determining if their personal objective is overridden by their camaraderie to the party, or the PC.
I think a lot of people are misreading Shadowheart's trustworthiness. The only reason we've never hit a serious point of friction with her is because nothing we've done so far as interfered with her objective of returning the artifact to Baldur's Gate. She mostly views our situation with detachment and disinterest (except for the temple). If we suddenly decide that returning to BG isn't a priority I think that would change.
