i've used a scythe quite a bit, and i wouldn't recommend clearing any land with it. but it works fine for cover crops, my partner and i have mowed several acres over the course of a week or two without killing ourselves. there are certianly crops better suited for being scythed than others, just consider the physics of it- the larger the stem and the harder the stalk, the easier it is to cut something down with. rye is a dream, oats and other grassy things can be a nightmare, and vetch is hell (it constantly gets stuck in the space between the snath and the blade and wet vetch can start to seem quite heavy after a few hours of swinging it around).
my friends who live in the next town over from me the wilsons have been making them for years and years:
www.themaruggcompany.com/ i've not used their super heavy duty blade so you might have some luck with that clearing land, but i wouldn't over sell it. remember these were designed to work in an era in which many hands were making light work, so unless you have a team of individuals all armed with scythes (which will easily cost you many hundreds of dollars in material and labor) you are probably better off using modern technology. the main reason i use it is to reduce soil compaction, which probably isn't an issue if you're working with raw land.
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