Although I am not advocating it, and would never try it with my longbow, a straight on shot is very deadly if the shot is good.
My father use to be a dang good shot with a compound and shot 80+ pounds with heavy aluminums and muzzy broadheads. He had a buck approach facing him and then the buck stopped and looked backward towards the way it came leaving the neck/chest wide open. My father was confident in his shooting ability and rightfully so. He drew, shot, and put the arrow right in the middle of the chest. That arrow passed through the deer and exited just infront of the left ham. He hit lungs, heart, liver, stomach, and intestines. The buck fell within sight, was gutted quickly and no meat was lost.
So it can be done and is effective, but you better be a good shot to attempt it.