Anytime you put a wing at the front of a projectile, you have the potential for steering. That is exactly what you are doing when you attach a fixed-blade broadhead to your arrow.
You work hard to figure out how best to shoot a bow and you tinker with the tuning until you have great arrow flight. Now, the last thing you want is an arrow that decides for itself which way it will go once it leaves the bow. With a wing at the front, there is always that potential. The larger the wing, the larger the potential problem.
I’m not saying a problem is guaranteed. When you have a well-tuned bow, a perfectly straight arrow, with a nock, insert and broadhead all in alignment and when you hold good shooting form through the shot, you will shoot most fixed-blade heads accurately. But if any of those elements breaks down, you will have a wind-planing issue. And the faster it flies, the more it will wind-plane.
If you remove the wing from the front of the arrow, or reduce the size of the wing, you eliminate or reduce the possibility for a problem. That is Aerodynamics 101.
The goal then becomes a combination of two tasks. First, make the wing as small as possible. Second, get the bow, arrow and your shooting form as good as possible. The smallest wing is no wing, and that is the only reason to shoot mechanical broadheads.