Achane is a damn good football player, and he went to the perfect offense to maximize his skillset. McDaniel is a genius playcaller, and the Shanny run scheme has a home for those super fast, Mostert-esque speed backs.
With that being said, his size limitations are stark. He can't run up the middle in the NFL, and he's never going to handle a high volume of touches.
I think he is going to have a young Tony Pollard type role, where he looks crazy freaking explosive for like 5 touches a game. I just don't think he can expand beyond that and become a present-day Tony Pollard. Pollard is big enough to horse it up the middle, and really developed as a pass blocker. Achane just doesn't have the potential to develop into that complete guy.
I think Achane will be IRL good, but not volume stat, fantasy football good. The best pro comp I've heard for him is Leon Washington. Kind of a really good changeup guy who was limited to about 100 touches per year.