After using both, one for many years, there is really no comparison. At least not for me. If there is a downside to the flail it's the fact that it doesnt cut well in reverse.
Kubota b2650, bx2200, L3940 (gone), New Holland FWD TN85, RTV 900
I've had both and I sold my flail. It was slow to cut and a pain to sharpen. The only thing I liked over a brush cutter was the ease to transport on a trailer due to its smaller size. If you want it to look neat, get a finish mower and cut it more regularly.
John Deere 950. LS g3033. Kubota RTV 900. Exmark Lazer Z
I have a brush hog that works fine, but it’s been parked for a few years. I cut my fields when growth is no more than 8 inches, and the flail mower just does A better job, and is easier to maneuver on my place.
I actually have 2 flails, an old fine cut Mott, and a 3 year old Caroni, rarely sharpen either of them, but do occasionally flip fine cut blades over, or just replace them.