My impression had been that quality/stars measured current skills (which would translate to immediate results on the field) and that value took quality/stars into account but also added in this potential for future growth (fast training) and how long it may be applied (age).
But instead I am hearing that quality and stars are about a potential that may be realized (fast training) or may not be realized (turns out to not be a fast trainer or despite high skill levels does not produce on the field)? And that on field performance can only be determined by actually playing a player and can't be estimated by any of the numbers observable for transfer listed players? This would be difficult to manage with. Maybe quality is still the best indicator of on field performance, but it is just not guaranteed?