I did have an old allen scythe once, and whilst it would go through most things I'm not sure they're any better or faster than a good brushcutter.
I decided to sell mine when I moved here even though I have a lot of land to clear - the land here's very steep and the scythe is quite a cumbersome, heavy thing to manoeuvre. Also, mine was one of the early ones with the clutch 'the wrong way round' - i.e. you had to squeeze the handle to make it stop - so if you got tangled in cut brambles and fell over it would go off on its own, circle round and come back at you.
On the other hand, if you can pick up one at the right price, (mine was £12 - the engine was in pieces in a box) they are effective.