Well, FIP assumes that all in the park contact is the same. And that works for most pitchers. Generally speaking, over a long enough period of time, there is enough consistency in the talent of MLB pitchers that all in the park contact normalizes somewhat and FIP becomes a reasonably good predictor of future success. My problem with the usage of FIP is the assumption that it works for all pitchers. It doesn't. Like most predictive stats, there are outliers who routinely under- or over-perform their "talent" (for lack of a better word) and kinda break FIP. I also dislike the use of career FIP once a pitcher logs several seasons of play, especially if said pitcher splits their career between multiple teams and leagues (thereby diminishing the impact of good/bad team defense and park effect). At that point, ERA becomes a more accurate assessment of that pitcher's ability because it (mostly) tracks what actually happened, not what was supposed to happen. But over- and under-performers of FIP appear to be relatively rare (Santiago and Nolasco are good examples of each), which means FIP is still a pretty useful tool, IMO. But, like most other metrics, it needs to be used correctly and it needs to be acknowledged that the metric doesn't work for every player 100% of the time.