I agree that there are concerns about how the offense will perform. There are a lot of question marks and nobody really knows how it's gonna go till the team plays a real game. The talent is there for success, it's a just question of whether or not they will get it together. As for some of the concerns mentioned:

Mangum: He played only one full game last year in terrible weather conditions and played mostly mopup duty in a couple of other games. He did come in the Utah State game when Taysom got injured but BYU pretty much had that game in the bag so he wasn't called on to do much. There just isn't a big enough sample size to get an accurate picture where Tanner is at running that offense in a real game. And the spring scrimmage hardly counts. Tanner is a competitive QB who expects a lot of himself. If he struggles, I doubt it will be for lack of effort.

Running game: There is no replacing Jamaal but that doesn't mean the running will suck. It will be a RB by committee by we have a good mix of players that bring different skills. If the O-line does their job then I think the running game will be fine.