I think Izalith is really cool. The map design, the art direction, the enemies position… I don't know, by the time you arrived that place, you should be prepared.
The only thing that was annoying to me was the Ceaseless Discharge. I've beat the game four times and this is one of the most problematic bosses to deal with if you don't know the "cheap trick". After knowing it, it's easy peasy, but it's still a bummer.
I think it's not a big problem to me because it's still one of my favourite videogames of all time. But I can see why people don't like Izalith.