I think they have potential to change their ways. It's not a new thing, even in bg2 companions could change alignments. Vioconia was really bad initially. This is a character I remember when I got her thinking what the hell but you could keep her even if you were good and eventually she becomes neutral. So I don't see why shadowheart can't. Spoilers from here - so far I learned that she loves Shar but one significant detail (that is even written in the journal entry) is the fact that she gave up some of her memories. My guess is that those memories will factor in her "alignment shift". I don't see what's wrong with it if it is well written.
Funny enough, Viconia switch to worship Shar from Loth (she is a terribly written cleric of Shar by the way) and her alignment shift is kinda bullshit. It's there because of people like you, who wants to keep a cool looking char in the party, but can't stand they complains all the time when the party reputation reach 20...
Shadowheart is already more "true neutral" than "neutral evil" anyway. She's more annoyed about breaking neutrality than good vs evil choices. And she is a properly written Shar cleric (they are a very secretive bunch).
