I say underrated.
As much as I liked Diggs, he is the only major loss in the off-season, and I do not believe it is as major as many think. By the end of the season, the Bills were one of the top four teams in the league; the offense will be different and equal, if not better than after Brady took over.
I also think the Defense will be better (Milano back, Douglas a full season). But I concede: The one big question is what happens at Safety? If we get equal play there from the newbies, the entire Defense will be better.