There now exists a capping effect for stats; you can now only reach +12 max to any given stat from the following sources (total):
- Item Effects
- Youkai Skills (including Affinity)
- Status Effects which increase stats
For example, if you were to have Mystic Affinity at Rank 5, and installed Carbuncle with 777 activated, you would get 7 (from 777) + 5 (from Affinity) to all your stats, giving you a +12 bonus to them. If you had any item effects that also increased those stats, they would be ignored, as you've already hit the cap.
Note that this doesn't apply to Install's racial replacement effect, nor does it apply to any inate or passive class skills that increase those stats, nor do those stat increases count towards the cap.