The problem is that it's stats driving development, which is backwards, imo.
Players do have ceilings. Everyone isn't good at everything "if they work hard enough" or "play well enough". Some of these are physical, even if related to body type or training due to position they play or role/techinque within that position), some are mental or "just there" for lack of a better way to put it. Why doesn't every player run like a 99 SPD, 99 AGI? Why doesn't every player have 99 STR or run routes like Jerry Rice?
We don't know where the ceiling is, and that's one flaw of the potential system - no uncertainty and no response to things like scheme changes, coaching changes, personnel changes, injuries, etc, all of which can alter a player's projected growth/decline and, thus, production. Another flaw is that it's "global" to the player. He might be A potential in STR and POW, but D potential in AGI and MCV, and C in TAK, etc.
The thing with the XP system is that it's unrelated to what a player does. How does making tackles make a DB more of a ball hawk? How does a pass rushing OLB become a run stopper with getting sacks? He is what he is - and being good at what he is doesn't make him better at what he is not.
Neither potential as presented in most games nor this XP thing really capture how players grasp new concepts or improve their weaknesses. I like the concept of the coaches having some sort of system for improving their players' development (after all, that's one of their roles, especially for positional coaches), while also having a performance sort of system to represent how some guys fit some schemes/coach's strategy better (kinda what M12 did but more robust and NOT impacting the base player skill).
I also think players need new ratings, even if they are "hidden" (as much as I HATE hidden ratings because no team is ever totally blind on an aspect of a player - there's just outright guesses and guesses with more evidence to support them). After all, not all players learn/adapt at the same rate, be it lack of work ethic, stubborness, or just can't get the hang of it.