Any (ok most)fixed blade cut on contact will be more reliable than any mechanical BH. Thats a fact and any argument is more opinion than fact. The reason is a mechanical has moving parts that can fail to open. An magnus cut on contact head is already open...nothing to fail.

That being said I am not a big fan of mechanicals. I have some rage BHs though and they are nice. I have yet to use one on an animal but its hard to argue with there success and blood trails.

If I had to choose one and everything else was equal (flight of the arrow) I'd go with a fixed blade everyday.