I like the fact that the bosses almost always have "tricks." Most revolve around Reaction Commands, but some are just wild.
It may not be "difficult" per se. Even Sephiroth was pretty easy once you knew how to handle him. But it also allows you to flow through the story. An even then, there are boss battles that are tough sheerly on a technicality (Demyx....ugh...)
All in all....at least rent it. It's worth a look.