I had a Gravely with a "bush hog" mower on the front and also had one of those DR trimmers on wheels (not self propelled).
I could not handle the (12 hp) Gravely until I put some steering brakes on it. Then, its main problem was traction to get up and down the back hill. My 4wd JDs with a bush hog on PTO are much easier to move about and have been far safer on hills and slopes.
I was always wishing the DR trimmer was self propelled.
Unless you're like Arnold S., a 4wd tractor is a lot easier to work with. Yeah, it takes a few passes at a tree to really trim around it, if you're that particular.
Ralph