There's nothing "wrong," exactly, with the Chemical Plant Zone. It's not especially hard or cheap or broken. But Merriam-Webster defines "terrible" as "exciting extreme alarm or intense fear," and by that definition, Chemical Plant Zone is one of the most terrible levels of them all.
Have you ever wanted to send a gamer into blind, slavering panic, reducing them to huddling, white-eyed, in the corner of a darkened room? Have you ever wanted to stop a gamer's heart for fifteen seconds and see their cracked lips form a silent prayer for mercy? Have you ever wanted to give 'em a good scare? Play this music.
Like running for exercise, nobody actually enjoys fighting the Flood, the parasitic space zombies of the Halo series. You enjoy the creepy build-up ("ohshitohshitohfuckohgoddamnitshit"), the first few moments ("OHSHITOHSHIT") and the relief and pride you feel when you're done, but the actual bulk of it is an agonizing slog through too many ammo clips and interminable dark corridors.
Nowhere in the series is this worse than the Library level of Halo: Combat Evolved, in which you've set out to retrieve an ancient artifact from an abandoned installation. Standing in between you and your goal is only all of the Flood in the world. With the Flood coming at you from all sides, finding the right path through the endlessly repetitive gunmetal grey hallways becomes a Sisyphean ordeal, forcing you to fight wave upon wave of skittering baddie when all you want to be doing is driving Warthogs on the beach. Worse, what creepy or tense atmosphere that might have been achieved is ruined by 343 Guilty Spark hovering around, humming and mocking you adroitly. Halo is a lot of things, but survival horror? Not so much.
For most of Half-Life, you beat zombie scientists to death with a crowbar, fight backflipping black-ops ninja assassins, and navigate a futuristic research facility that's being pulled down around your ears. Then, after beating back wave after wave of murderous marines and alien scum, you travel through a portal to the source of the alien invasion, at which point the game goes from "Check out all these cool, constantly changing environments!" to "Check out this vast, empty void!"
Gone are the awesome three-way dynamic battles between the factions struggling for control of Black Mesa. Gone are the NPCs whose goofy dialogue and horrific deaths were a constant source of brevity. Gone are the airducts to crawl through, industrial equipment to climb on, and all the vibrant, decaying locales that made Black Mesa such an immersive environment. Instead, you get the emptiness of space and some jumping puzzles, which, frankly, are a lot more annoying when you can't even see your feet. No puzzles, no interactions, no plot twists, just hopping. The tedium of Half-Life's overlong last segment is made up for by an intense final boss and a super-creepy, super-memorable ending, but only barely.
Full disclosure: I actually totally enjoyed Xen, but I also totally enjoyed the Nikki and Paolo episode of LOST, so maybe don't trust me.