In my experience all the sharpeners all good, it's more the operator and his skill than the machine. Many mechanics are skilled using bench and hand grinders. I have a lesco at home and use a large hand grinder in the shop.
I take a air sander and first clean the blades then sharpen the blade with a hand grinder in a vice and finish sharping with a die grinder by hand then balance the blades. This is all I have ever used for myself and customers. Do these blade sharpeners work better? Would they be faster and worth the investment? It still seems like you would still have to clean and balance the blades? Also how well do they work when you have a blade way out of balance that someone messed up not knowing how to sharpen mower blades?
I have and use the Mag 8000 for years and I just love it! It is a bit expensive but well worth it if like me you get all kinds of different angles of blades. I have no complaints about the machine or the company support they are top notch in my opinion! The feature I like best is the quick change angle set up VERY EASY and Quick also easy contour blade change wheel and post!
I purchased a Yellow Hornet blade sharpener for $150.00 along with a $20.00 41/2 inch angle grinder from Harbor freight.
I used a bench grinder for years but I like to have my tool "toys" so I purchased this.
Once it is set up I can run the blade thru with 3 passes and I have a very accurate 30 deg angle
Very few sharpening machines can cope with anything other than a flat blade so they are limited to non mulching blades for a start ( Gators excepted ).
The angle is nowhere near as important as people like to think.
The draught is created by the flutes on the blade and the blade angle is the minimum that provides support to the cutting edge.
or long life & optimum cut what is important is the thickness of the flat edge.
Too thick, you get tearing and put extra load on the entire cutting system
Too thin then the edge gets deformed easily and wears away rapidly.
Like sharpening knives, soldering , welding, plaining wood etc, these are hand eye skills that you attain through consistantly doing it.
Once you start using jigs & quides you loose those skills.
I can hand file a chain saw blade, faster and finer with no guide than I can do the same job with the blade grinder.
How ever is there are 50 of them then the grinder gets plugged in.
This is a skill I developed over many years.
I have used a portable belt sander sitting on its back. I use coarse belts depending on how beat up the blade is.
With this setup I don't need to buy, or more importantly, store extra tools. If I want I can clamp the sander in my workmate for more intricate sharpening. I always balance the blades.