In some ways Milb is about being productive with the bat and in some ways it is about failing. It is also important to appreciate that not every organization operates with the same approach and philosophy.
If one would make the biggest observation about all levels of Milb, it would be organizations are most interested in development against the manner in which that organization measures development.
As an illustration, our son was not at all a power hitter in his organization. However, after a very successful season in A ball in terms of doubles and gap power, he learned the organization saw him as a potential 15 HR guy as he moved upwards, with certain changes in his swing and approach.
If one were to make general observations, they would include the fact that Milb is about development, not necessarily, especially at the lower levels and Rookie ball, "stats." A higher draft pick, especially out of college, putting up big numbers at a certain level, especially from Low A ball, may well get quick promotions upwards during a season (even 2 levels) until he reaches the point where he fails. At that time, the development and coaching truly becomes apparent and the hitter goes through the process of adjustments looking to prove he can succeed and get another promotion. Failing at Low and High A does not equate to "failure" in terms of release unless and until the organization is clear the hitter cannot adjust or unless that organization has a surplus of talented hitters especially at those positions where some are "failing." By the same token, success at A ball does not "equate" to success because nearly every hitter experiences failure which then leads to the development to the next success.
We really can never talk about this in the "vacuum" of a hitter because pitchers are going through the same process, but in a reverse sort of way.
Players from Short Season A may get some late season promotions but those are often times more organizational than due to production. Some organizations may actually demote some players if post-season play at the lower level and success within that organization is a consideration (very often it is not.)
Given the option, I tend to think most organizations want players to "fail" in their Milb system rather than have a trajectory of success which leads to failing at the Big League level. The Giants this year have 3-4 examples of rookies who had wonderful success all the way through AAA, only to struggle mightily at the major league level. One reason might include not enough AB's in Milb and no true failures. In any event, because of the failure at MLB, the book is now open on each player as a true prospect as contrasted with a AAA1/2 star.