I had a flail mower (basically a 60", think it was 64" or something around that size and a very heavy duty unit) with a 50hp tractor and some heavy brush. It would cut it but was SLOW and did a so so job and used a LOT of power. The knives were sharp and everything was in great shape. I then threw my 6' brush hog on, ran it over twice FAR faster and everything was shredded almost to the dirt. Sold the flail and have never looked back. They get a lot of hype but I just don't see the advantage unless you have a lot of concern with a brush hog throwing things outward. The nice thing is the brush hog is cheap to maintain and has little to break!
Not saying you shouldn't get a flail but wanted you to hear that not everyone loves them.