Something went south in his game partway through the season. Maybe he isn't ACC caliber talent, but he definitely dropped off. Quite possible he was just overwhelmed.
It's tough to gauge where he will be in another year or two. Some kids get better. Some regress. Brad was quite overmatched as a freshman and is now possibly the best professional PG not playing in the NBA.
Maybe Marcus just didn't fit Stallings' system. I'm not sure he's a lead guard, nor athletic enough and has shooting skills enough to be a 2. He's a decent combo who could be a valued reserve at both guard spots for a team starting over in building a roster.
At worst, he's pretty good depth.
These are fair points....
I would counter he would improve dramatically with better talent and finishers around him.