Can I sub buckwheat honey for molasses?

Honey and molasses do not at all act the same in baking. Honey's natural enzymes denature protein. Molasses is sugar, cooked down. While I agree that the flavour profiles are similar, substituting is a bad idea.

Don't sub it for molasses -- they're not that similar and molasses is A LOT cheaper.
I mostly eat buckwheat honey with butter and salt on toast. I've never been willing to sacrifice a whole jar to this recipe for almond-honey squares, but I have had the product and it is delicious: http://www.davidlebovitz.com/caramelized-almond-honey-squares-recipe/

Buckwheat honey has a very pronounced flavor; taste before deciding about its use in a given application.