If someone's gonna say that fantasy sequels inherently suck, they'd need to bear in mind that The Lord of the Rings was originally requested, and intended, as a sequel to The Hobbit.

The Final Fantasy sequels are what we make of them. And what Square-Enix makes of them. A chance to expand a story, continue it, or even examine it from a whole new perspective. A sequel has the potential to go well, or go badly - but overall, I'd say there's a marginally higher chance of it going badly, since there's the built-in tendency to rely on the popularity of the previous work. It's not often a sequel needs an entirely new cast and new world, after all, so there's less 'creativity' going into it by default.