Mechanicals shot from a well tuned bow into a perfectly broadside animal are fine.

The problem arises when the arrows and bow are not tuned properly and the mechanicals become a bandaid fix on a bigger problem . (IMHO, the most common reason that mecahnicals get used is when the hunter is having flight problems with his fixed blades)

The kinetic energy of a 400 grain arrow is something that can be degraded very quickly by unstable flight or by uneven resistance when the arrow hits the animal. Some mechanicals are better than others but the mechanics of opening that head uses some of that KE. Angle hits from an unstable (wobbly) arrow take a whole lot more precious KE away that could be crucial to adequate penetration.

Just my $.02