So if you still get a good hinge is there any problem with an angled back cut?
-Angled back cuts reduce/eliminate the effectiveness of wedges.
-It's much harder to determine where to start your cut in order to achieve proper intersection with the face cut.
-If you do miss (which is likely) and cut through the holding wood, it's more likely to jump off the stump and fall unpredictably. Especially with a Humboldt face cut.
-Could potentially cause a tree with a head lean to barber chair as you have to cut more wood to achieve the same thickness of hinge...meaning the tree may leave before you want it to potentially exacerbating barber chair issues.
-A back leaning tree or tree that sits back against an angled back cut puts more cross grain shearing force on the hinge wood in it's weak direction and could cause the hinge to break where it wouldn't on a flat back cut where the force would be more tension across the whole hinge wood instead of shear.
Angled back cuts have no actual benefit and numerous drawbacks. The 'reasons' I've heard for using them are that they prevent the tree from going the wrong way and it prevents the trunk from jumping backwards off the stump if it hits something before hitting the ground. That little bit of angle on the back cut isn't going to keep a back leaner from going backwards...ever that's just a ridiculous lack of physics knowledge. Keeping a stem from jumping off backwards upon hitting another tree isn't a problem with proper cutting technique. So both of those reasons are invalid.
So in conclusion, it's never advisable to use an angled back cut. Even if you do it perfectly, you're still handcuffing yourself and making your cuts more difficult for no benefit.