So our plan isn't for uniques to be objectively superior to rares (think Diablo III), but rather for them to do something different. Crafting items will be one solid way of getting a good foundation for your character, but you might want specific uniques which are build-enablers, or otherwise contribute to your build in an interesting way. Please understand that a lot of elements of the game are currently very much WIP, and balance (e.g. drop rates vs crafting) is top of that list.
Some uniques will be rarer than others - and we'll be making an official distinction where the rarest / most valuable are labeled Legendaries.
Appreciate your questions and the kind words!
Spot on, one additional thing to note is that some particular uniques will be exclusive to certain bosses or will have higher drop chances with their respective bosses, though these will not be the majority case.