I love the idea of the wraiths, and use them quite frequently out of personal preference, but are they really good enough to include in a squad?
They are the fastest non-dragon unit for waiqar, but at the same time their heroic doesn't work very often with only two dice, which leaves them at kind of a weird point. Good at everything, without being good at anything. Also I think wraiths prove just how overpowered the Riverwatch Riders are.