As I've written here a few times I've had an AM for years. Some points:
Teeth dull easily. So, either dig as much as needed (depends on level of rockiness of soil), or use dull teeth to clear all around the stump and roots.
Teeth are about $8.50 each, and easy to change. Can be resharpened a few times with a Greenwheel.
Simple to maintain . Just change teeth, clean saw filter, and run full syn mix at 40-1 or lower ratio. Belt rarely needs adjusting.
Two different teeth... the regular tooth has a more radical cutting angle, and thus dulls more easily. Therefore, I use more rock teeth. but, if kept quite sharp, the machine grinds quickly.
The machine is hard on the body, and does cause my hands to go numb... but so do many things, like sleeping on my left arm. The right never goes numb. I need to find a good chiro, or a hot massage gal.....
I average close to $160 hr with it, which includes time digging and changing teeth. After prep work, a 12" stump with a few roots will only take 6-10 minutes to grind to 10-12" below grade.
I don't use it much, as most stumps can be reached by my sub with a Rayco 50...
I don't have the transport wheels, which would be nice. I carry it in my pickup, so don't take the time to disassemble it.
I have mine powered by a Husky 3120, which I got used for $1100....included 43" and 72 bars, which added to the 42 and 60 quiver for my other woods ported 3120.
Largest stump I've ground had a total area of close to 100 sq feet. Prolly took 4-5 hours, got $900, iirc.