So why is it that people hate on Hoenn? Not gen 3, just hoenn.

Water routes, mostly. Right?
Well, while you do need an HM (sometimes two, occasionally a third), Surf isn't necessarily a bad one and one Pokemon can usually get all 3, if needed.
Encounter rates on rater routes are constant, in addition to the trainers spread out and pointless obstacles. At least Hoenn and Unova's longer ones had islands that occasionally gave you items, but the water itself is endless encounters.
But hey, tons of exp right? Well no, because the lack of variety means you're spending most of the time on the ocean fighting Staryu/Magikarp (owned by trainers) and Tentacool (wild), which don't yield a lot of exp, even with the boost from facing a trainer. And it means you're not expanding your dex because everything's the same thing over and over

Couldn't all these same things be said about cave routes? And Kanto had more than its share of those.
I drew up these maps for comparison, with the red highlighting the cave routes.