how do the levels stack up next to Hollywood Holocaust?
They don't. They're not distinctly setup and self-contained like the Duke 3D levels are. They're largely singular paths that take you through the campaign with a side attraction or two on the way (like picking a turd out of a toilet or playing a stationary turret segment) and little to no free-form exploration, only exploration where it is allowed (pretty much nowhere). I don't recall if the game even had any secrets...