From a playtest with 5 18th level characters against a Frost Giant Jarl + a Very Old White Dragon:
-The dragon has a ridiculously high CMB modifier, making disarm/sunder/trip attacks trivially easy.
-For high Strength monsters, the Pathfinder, all-or-nothing version of Power Attack is very painful.
-Accumulating miss chances (e.g. Displacement, Mirror Image) is much more useful than trying to improve your AC, so giving the fighter class extra attack bonuses is nice, but it still doesn't help against the most powerful defenses.