Pierre the choice between a conventional face and a Humboldt face is not to make a balanced tree fall or to start toward the face. It is what you want the tree to do once it starts to fall. A conventional face will keep the tree attached to the stump via the hinge until it hits the ground. That means good control and a fast falling speed. The downside is likely more breakage. A Humboldt, esp with a snipe (A second face on the lip of the first face) will kick the butt to the ground early or make it jump, depending on what is needed, and reduce breakage. This is esp important in big conifers like redwood.
Actually this is rocket science. It is just that the very complicated basics were developed many years ago and we just copy their good work. Reducing breakage doesn't matter when you are cutting fire wood or just doing a removal. For professional fallers it is their bread and butter. A fire wooding homeowner does not need to make a 200'+ tree jump a ravine but a timber faller does. Heck, for me a quarter of the trees I fall really do not need precise directional control, just get it on the ground safely so I can drink a Red Bull and eat some steaming Brussell sprouts out of my thermos. The rest need to hit the ground in a precise location or my insurance cost will go out of sight. That is where the rocket science comes in.
Even if you never use anything but a conventional face there are many, many modifications that can be applied here too. Variations in holding wood, Dutchmans, boring, pulling, etc. As Hammer said there are times when your face or backcut may not be placed perfectly flat. You may have to leave little or no holding wood, that is cut through the hinge. Go to Bailey's website and get a couple books on the subject. They will help with your decision making process and maybe make your day more productive.