I could sit here and talk about how gorgeous the levels look, but you'd be much better served by just loading these bastards up and playing through them yourself. Really though, apart from the stuff mentioned and I guess the red key fight from 'Unstable Journey' (since I didn't grab the BFG first), it's not nearly that bad. As a whole, it may be more trappy than you can handle, in which case I'd dial it down to HMP. 'Perhaps Now the Vultures' is another exacting level whose glut of arch-viles will put a metric ton of pressure on the player as they sprint through the map, resurrecting previously benign monster corpses. I'm sure more skilled players could work out the 'hard' way, but screw that. 'Steeple of Knives' has a precarious balance which all but requires a certain order of visitation lest you find yourself beset upon by creatures of the deep. There's also a full-on slaughtermap in the secret level slot named 'Fireking Says No Cheating' and it owns bones, even if it kicked my ass all over the place.
Things start out pretty mellow but the difficulty ramps up considerably as you play on and the more open areas make for plenty of snipers.