I'm not adding much to the many mixed reviews here - and there's several who've written longer and more detailed comments that I agree completely with.
There's, theoretically, lots to like here - the new weapons are pretty good (the railgun shoots through glass allowing for some nice integration with level design too), the design on most of the new enemies is great (I think the tridenty guys work best, the chaingunners feel slightly off), and there's tons of atmosphere about the levels, and not just from the cool rain effect in the second map. (I really like a lot about how much character and sense of place the entire episode has, in fact, it's really really strong.)
However.... yes, the later maps especially are far, far, far too long and labyrinthine. I managed to just about get through map 4 - although I got lost and turned around a lot, even with the tweaks that may have been added in the later released to reduce the need for backtracking (I'm assuming that's what those teleporters near some keys are...) - but it was really pushing my patience and engagement by the end. It took a lot of goes to manage the timed escape sequence because I just couldn't work out how to get to the second level in the first place for a long time, let alone navigating around it when being attacked - please tell me at least the time limit is longer on Easy?.
Part of this is, as others have noted, there's a strong dependance on "there's a split path for keys" in a lot of the design, which makes it harder to remember where you actually are this time, as you can see lots of motif repeated. Part is just that the levels are too big!
Admission: I didn't finish Map 5 - it's also huge, it has an even more confusing design principle yes, it's very cool that we have the parallel world maps, and I recognised that they're tonal opposites and you're probably supposed to find the tomb in the red world is not empty when you get all the gold keys but... ...but, I got to the point where I'd found 2 keys of the many I needed to, and just could not work out where else I was supposed to go, and just kept revisiting old places and getting turned around.
(It also does not help that there's a sudden increase in jumping-traversal in Map 5 - and yes, you do take damage when you fall off into the void and are teleported back - which I am not good at.)
So, I level skipped to Map 6, the Boss level which... well, again, it's lovely, the homage to START is really finely judged... but the actual boss fight is incredibly frustrating. I mean, I worked out what to do, but it was incredibly frustrating to even do the first phase (and eventually I needed to turn God on just to get past that...). The second and third phases I just kept God on for, I would not have completed otherwise.
Yes, on easy.
(In fact, the combat balance is fine on Easy - it gradually ticks up over the levels and was getting about right for me on Map 5 - except for the boss fight which is definitely too hard.)