I have shot both mech and fixed blade heads off and on over the years. Despite shooting a bow that is only hurling a 380ish grain carbon arrow at about 260 fps I have only had one mechanical head fail to pass completely through a deer. That was a 230 lb 11 point and I hit the offside shoulder bone squarely on an angled shot. Quick note: I have had several fixed heads not pass through deer because of hitting the offside shoulder as well. Didn't matter they all died.
In my opinion people have problems with mechanicals for two primary reasons.
1. Some use them as a quick fix for an untuned bow. A mech head may hit with your field tips no matter what but if your bow isn't tuned you are still bleeding power as your vanes work to stabalize your arrow. TUNE YOUR BOW, or have someone do it for you that knows how. Good arrow flight or the lack of it is the reason people with very similar power levels often get such vastly different results using the same head.
2. People use a size cutting diameter that is just too big for the power level of their bow. Don't listen to any BS about the IBO speed your bow is rated for. Almost no one actually gets those speeds from a bow set up for hunting. Shoot your bow through a chronograph, weigh your arrow and calculate your level of kinetic energy using the FPS and weight you get. Many think that it is the energy needed to open a mechanical head that reduces their penetration. I disagree. The power needed to open even a fold from the from head is minimal. The real culprit in my opinion is the cutting diameter of some of the heads out there. It just flat takes a lot of energy to push a 3 blade head with a 2 inch cutting diameter through a deer. It would take a lot of energy to push a 3 blade fixed head that wide through a deer too. It's just that no one makes a fixed head that big because of how hard it would be to tune from a modern compound.
When some people head to the woods guilty of both of the above some truely abysmal results often follow.
If your bow isn't generating kinetic energy levels up at least close to 70 ft lbs then stick to a mech head with a cutting diameter less than 1 and a half inches. Two good ones are the Rocket Steelhead and the 1 3/8 cut Grim Reaper. Grim Reaper makes a larger cut head as well so take note of which you are ordering.
From almost any modern compound that is properly tuned a good mech head of modest cutting diameter will give good service.