My Peruzzo 7.2" is 1,500lbs. My L4060hstc is probably under 5,000lbs with loader & loaded rears. If I side shift it over. It torque my machine pretty good. That's just side shift with only 1/3 or so of the flail hanging out to the side. Not even an offset one like the other guys are talking about.
I'd look at an old sickle that can run vertical for your use case. A sickle also slices cleanly. A flail is pretty safe on the ground, but raise it up in the air & the spinning hammers or knives is going to have the ability to throw debris around a lot.