To understand why FFXIII was a huge let down take a look at this
Checked out FFXIII
Look for a member named Tibiquera has a long detailed post about why it was major let down.
As for KH2 vs. FFXIII in which was worse hate tips to FFXIII I don't like KH2 for various reasons, but their minor compared to FF13.
For KH2 it had a very cheesey storyline I'm talking dialogue wise and plot holes, and the fact that they seem to forget about the plot after the beginning up until midway around the 1000 heartless battle then the plot seems to disappear again somewhat. Then the battle system it's way to easy the entire time except for a few reaction commands you could mash X to win it didn't take any skill maybe besides a few dodges, but the boss battle's were lackluster even the final boss. I was mad with KH1's final boss battle, but 2's was worse it was longer and looked cooler, but worse.