The buckwheat honey I've tasted was VERY dark, almost black & looked for all the world like a thin molasses. It had an interesting flavour, a bit molasses-like, but the sweetness was mixed with a sort of flavour that reminded me of they way ripe hay smells; not what I'd call "grassy," but others might. To me "grassy" tastes like lawn clippings.
Others have called it a "barnyard" flavour, but I'm still not sure exactly what they mean by that. Think honey & molasses with toasted oats & ripe hay all rolled into one amalgamation of flavour & you've got buckwheat honey.
If you want to make mead with buckwheat honey, I'd mix it roughly 50/50 with a nondescript honey like clover or alfalfa. Less intense flavour, but all the honey goodness you want/need for your end product. Use yeast nutrient/energizer & DAP, you should have a tasty mead in a couple years.
Regards, GF.