I may have this wrong, but what I recall happening with Unga was that he was planning to come back for his senior year at BYU, then he got busted on the "Honor Code" violation and chose to withdraw, however this was after the deadline for declaring for the draft. So his only option was the supplemental draft and that's where we got him for this year's 7th round pick... i.e., 2 picks away from the last pick in the entire draft.
Scouting reports on Unga are hard to come by but it appears if he had entered the regular draft he would probably have been a 6th round pick or maybe a 5th, not so much based on production or character but based on the fact that he's a 6'0 240-lb guy that's less than a franchise RB but more than just a blocking FB so a lot of scouts had problems seeing how an NFL team would find a way to use him. I think it's very likely he's not so much a project as a "tweener" and/or specialty player and those kinds of players tend to fall in the draft, sometimes severely.