No experience with the FAE tractor attachments, but I've spent a bit of time driving other forestry mowers. How good they are really depends on what the intended purpose is for clearing the land.
The biggest complaint I have with every single forestry mower I have used is chip size. There's no uniformity, with some chunks being near sawdust, and others being chunks nearly a foot long. Of course, this can be helped by travelling slowly and being on the top end of the horsepower curve, but there's no real way to get uniform results.
I definitely agree with barpincher in that when the land needs to be cleared down for planting the mulcher/mowers do not do the job sufficiently by themselves.
Of course, if you are dead set on getting the mulcher and need to be able to clear for planting, there's a lovely type of farm machine available. A derocker or rock picker is a pretty standard item for a farm implement company to own and rent out or offer as a service, as you generally only need it once per field. There are quite a few models out there, the one I have the most experience with is a Rotoveyer, made by
Rockland.
Last time we cleared a field, we rented a forestry tiller from a local timber company and used that to mulch the land cover and then another pass with it set as a tiller to loosen the ground down to a foot. (AFIK, there's no rock clearing system that doesn't need the soil to be at least somewhat loose.) After the ground was tilled we brought in a 160hp tractor with a Rotoveyer, rented from a local potato farm. The Roto picked up pretty much everything that wasn't dirt. Roots, rocks, chunks, etc, I was quite impressed by how small of material the Roto picked up.