I had purchased a ranch with 150 hectares (about 370 acres) with a eucalyptus forest. The forest had several eucalyptus species (red, white, globules, maidens), of which red is the hardest.
We were able to clear all of the stumps, but the red did take more time.
The key factor was stump diameter; It works well in stumps with 20 inches or less diameter. If they are bigger, it is not easy to remove the stumps.
We worked with a 140 HP John Deere tractor, and we were able to clear about 1 acre/6 hour. With a bigger tractor, you might be able to go faster.
The stumper is virtually maintenance-free other than the blades. A set of blades lasted about 20 -25 acres.
If there are many rocks in the soil, these will break the blades.
Also, the blades need sharpening every three or four hours.
The works are tedious, and it does take a lot of person-hours, but the results are outstanding.
We have used the cleared land to implant pastures (ray grass 7 white clovers) for cattle and grow sorghum and oats for cattle feed.