I've always heard that low was bad, angled is worse.
Low and with a saginaw (conventional) can cause a kick back, where in the tree jumps backwards off the stump, you know at the faller..., using the humboldt negates this for the most part.
Crooked back cuts that cross the face, can cause the tree to fall away from the desired direction, similar to leaving more holding wood on one side or the other,
Crooked back cuts in the sloping toward or away from the face way, are mostly just sloppy, but if steep enough can make wedging a real nightmare, because instead of lifting the tree, you are in effect trying to push it over, this can lead to the holding wood breaking and having the tree choose its own path (i.e. very bad). It also makes pounding the wedges in difficult as Hel, especially if it slopes in an upward direction...
Having the back cut to high can lead to lots of fiber pulling or a premature holding wood release (cutter keeps on cutting and wacking wedges and before he know it he/she's past the face cut and the holding wood just splits off like cord wood...)
There are situations where you could use anyone of these methods for your benifit, but there are other and safer ways of accomplishing the same thing...
I usually try to have the back cut flush or up to 2" higher than the face cuts, and the back cut parallel with the face, unless its a soft dutch or some other fancy face then its perpendicular with the tree... ish... kinda... when I'm lucky... and not to tired...