I am always concerned about the possibility of a sharply quartering shot with the mechanical broadheads. They have the potential to not open at that angle. I probably would have passed on a nice 6x6 bull elk a couple of years ago had I been shooting a mechanical rather than a fixed blade. He was in his bed and I had to shoot from the rear and slip an arrow in behind the ribs, angling to the front opposite shoulder. The broadhead ended up against the hide behind that off front shoulder.
I also am concerned about wasted energy in the "opening" process of the mechanical broadhead on a large animal. I have used mechanicals, but on smaller animals (antelope, deer, coyote) and they are very deadly. On larger animals (elk, moose, bear) though, I stick to the 100 grain fixed blade NAP Thunderhead for insurance. I have seen the videos of the mechanical's performance and they're impressive, but a lot of bowhunting is what you feel good about and have confidence in. I am just funny and like Thunderheads on large animals.