I can't imagine that would be comfortable to use unless you managed to find one of the backwards tractors (former all terrain forklifts??).
That company does have hydraulic (skid-steer) cutters that would be easier to use, but also somewhat more expensive. It may be able to be mounted to a front end loader if you had adequate hydraulic flow.
What are you doing with the tops of the trees and the stumps?
How many acres?
About 30 years ago my parents hired a dozer to take out about 5 to 10 acres of nasty wild pear trees in a pasture. I think he was done in a day or two. But, left a dozen big piles of trees to burn. It took us a year or so to burn all the piles. A bit of a process of burning once. Then taking a Pulaski to chop the dirt out of every major stump. Then repiling and reburning. Perhaps 3 burns per pile.
Hmmm... we've had about 30 years of a good hay pasture... perhaps I'm forgetting the work in burning those piles!!!
Nonetheless, I think it was worth it. They also hired a commercial brush hog (excavator mounted?) to do an adjoining pasture, but unfortunately that pasture has completely regrown as if nothing was done to it.