First is the question about standards. Who will determine standards of proficiency, particularly as they pertain to specific disciplines. Faculty have historically been concerned about academic freedom in their classrooms. Within disciplines, often there are disagreements about philosophies and priorities. Second, a proficiency-based system significantly changes evaluation systems as we currently practice them.
Judgments concerning proficiencies are incredibly time- intensive.
It is easy to imagine that as much as one-half the educator's load would be occupied with evaluative judgments to make the case for proficiency. It's called practice, practice, practice. At the lesson comes the opportunity for periodic evaluation and adjustment. The cost of proficiency can be prodigious, naturally. A factory which produces Fords will never by mere reallocation generate a flock of Ferraris.