What exactly are planning to use a flail for?
A lot depends on the particular brand and model of flail mower. Mott invented the flail mower and are now owned by Alamo. These flails are designed for heavy grass and weeds. They are not designed for brush cutting. For grass cutting they use less HP than a similar size rotary mower. They are much safer in that they don't throw anything out the side or back like a rotary. The cuttings are thrown against the deck and then fall to the ground behind the roller. There is maintenance involved that includes greasing the bearings in the cutter shaft and rear roller. Changing the knives is more time consuming than changing blades on a rotary mower or bush hog. Compared to a single blade bush hog a flail will leave a much nicer cut and mulch the cuttings a lot more as well.