In my opinion, and keep in mind, this is only my opinion based on 45 years of bowhunting. The biggest issue with mechanical broadheads, and there are MANY issues, is the blades deploy at steep angles without much sweep. The steep angles of the blades don't allow them to slice but instead chop. The long sweep of most fixed blade heads allows them to slice by means of a mechanical advantage. The longer the sweep, the more mechanical advantage and the less energy required to slice it's way through an animal. A 3:1 ratio of length to width is about the perfect ratio. Slicing through tissue allows for very clean cuts of the blood vessels which increases bleeding.
Chopping through tissue tends to rip the blood vessels which promotes the clotting of blood at the ends of the damaged blood vessels shutting off blood loss before stopping it completely. Kind of like a farmer that gets his arm ripped of by a piece of equipment out in the back 40. He gets himself back to the farm house, calls 911 and waits for an ambulance to take him to a hospital to get his injuries attended to. If the same farmer had that arm sliced cleanly off by something razor sharp piece of equipment, he probably wouldn't make it 100 yards before dying due to blood loss.
You can test how the different blade angles work yourself. Next time you eat a steak, try cutting it by holding the knife at different angles when cutting through your steak. Try cutting your steak with the knife held at an angle similar to the angle of your mechanical broadhead blades. You'll see that steep (somewhere between 45 and 90 degrees) angles don't slice through your steak very well at all. You can force it through the steak with enough force but you'll end up tearing the steak, not slicing it. Now lower the angle to about a 30 degree angle and you'll find the knife slices easily through the steak.