The whole concept behind Mechanical heads is they fly like your field tips. They are a product designed to compensate for improperly tuned bows, and unaligned heads to shafts. Alot of archery hunters don't understand how to make sure their equipment if properly tuned, and setup. If your equipment is perfect, most fixed blade heads will fly properly. You get less windplanning problems with field points, so quite often shooters don't realize they have a problem with their bow setup. They try a set of fixed blade broadheads, and the problem shows up. So they think the fixed blades are junk, and switch to mechanicals. Low & behold they shoot like their field tips! So they swear by them.
The original knock on mechanicals was they didn't always function properly, and wouldn't open up everytime. New designs have solved that mostly.
The next knock was at angled shots like quartering away shots. The way the blades were designed to open at impact. A side angle hit, would kick the arrow out once it made contact. This resulted in less penetration. Again most modern mechanicals have solved that.
The present concern is not getting as large of a cutting area as fixed blades. The blades only open so far, and don't give as much cutting diameter as fixed blades. Some companies are recognizing this, and making designs to get more cutting surface.
The best thing on fixed blades. You never have to be concerned about not working. Obviously some designs are better than others. But the blades are always open, and cut at all angles w/o lack of penetration concerns. Their seems to be a larger interest in cut on contact broadheads, and with good reason. They work well. Broadheads have always been about sharp clean cutting to cause lots of bleeding for a quick clean kill. So it makes sense to use a broadhead that cuts immediately on impact.
Others have concerns of hitting hard tissue,bone at first impact so perfer a head with a tip that can penetrate such hits. I've always seen this as a setup for the archer thatpulls heavier draw weights, and uses a heavier shaft arrow.
What it all boils down too is your setup, and what your looking to accomplish. Their are alot of good heads outon the market now in both fixed, and mechanical heads. Do some research, test shooting, and determine what is best for you, and your setup. After all thats a part of archery.Its a personal perference, and what works for YOU!!