If you tune your set up:
Fixed heads- fly just as good as field points, have increased penetration
Mechanical- fly just as good as field points, have less pentration, increased cutting dia.

Good hits with both heads- dead deer within 100 yards, deer shot with mechanical falls 3 steps closer than deer shot with fixed head.

Bad hit on a BIG deer, not a pissy 4 pointer- Fixed heads, with the increase penetration will perform better, perhaps enough to find the poorly hit animal. Mechanical head without the good penetration less violent wound, less likely to find the deer.

Un-tuned bow
Shot placement is the most important aspect to bow hunting. If you have to use mechanicals to get the accuracy I say go for it. They are a quick semi fix to poorly tuned bows.