We have a large version of a flail mower called a cutditioner
It's a great piece, pulls behind, easy-ish to care for...
HOWEVER, it does not like rocks or like items and I've had it launch stuff out the front. If you get sometihing like this, I suggest you wear a helmet. If its front mounted, I suggest you make everyone else wear one.
The swinging blades/teeth/tines will give like a brush hog when in contact with a rock or immovable but you have two main differences. It creates lift because of the rotation (a big benefit if the hay is down) and instead of one cutting edge to sharpen, you've got 50 or more.
I've no doubt that a flail will do the job, but it's my opinion they are designed for clean environments with just foliage to deal with, not downed branches or other items that could be an issue.