I can certainly see the effort put in here, and everything is overall fairly impressive from a technical and visual standpoint. That said, I just don't really vibe with the current direction much of the game's level design has.
The best way to describe it is that it feels confused. It wants to lean into more momentum heavy level design and explorative, nonlinear level design, but it also doesn't feel like it commits or does it very particularly well. Despite being big, it doesn't quite feel like something like the vanilla game or even some mods I've seen, where there's clear marks between higher and lower/faster or slower routes or even pathways that lead to totally different portions of the stage. If anything, it's they can feel rather directionless. I've found myself getting lost at points in even the first few levels because things don't feel signposted very well, and there's sections which just have a bunch of stuff in it to distract you from the one hidden part tucked away you actually need to go into to proceed. Stages like Tempest Valley and Verdant Forest are especially bad with this, with some of the level design feels like it's made more to look pretty and intricate as architecture over being actual fun levels. As a result, you'll get things such as the villages at the end of Verdant Forest Act 2, which make an already extremely dark and hard to see stage even more annoying as you fumble your way and try to find the exit. This can also mean that since a lot of areas just feel open with no particular reason beyond to waste your time, the stages themselves can still feel quite linear in places. It's hard to put into words, but the best way to describe it is that some levels such as the ones I listed above look massive, but actually feel way smaller and the greater size just makes them more of a chore to play if anything.
Some of this wouldn't be as much of an issue either if the level design just didn't feel so boring. As said before, the game clearly wants to emphasize greater usage of physics and momentum in the level design, and levels themselves have been scaled up to accommodate. The problem is I don't really think it plays into this sorta level design much at all. There's slopes, sure, but ramps and steep inclines or general things that could be used to feed into this sort of level design feel few and far between except for specific key sections. A lot of things can still just feel flat terrain wise, yet since the levels are still designed to be so big, there's wide stretches where there's just nothing interesting going on.
Sonic stages like these work best when they feel really dense; never really having too much totally flat ground unless you're intended to be in a state where you can really go fast through it all, or there's some interesting platforming going on. SRB2 vanilla does this pretty well I think, with many stages feeling compact with a lot going on, even if it doesn't really focus on the momentum aspect as much currently. I don't really feel like the stages in Mystic Realm really do that, and as a result you can just find yourself trekking through these big, wide portions of stage with nothing of note.
Not that the momentum portions really feel like they work too well to begin with. I'll often find myself struggling to get up ramps even with a fully charged spindash because the speed loss going uphill feels just so strong. The fact that SRB2's insanely rigid rolling steering haven't been adjusted at all either can make a lot of sections which feel like they were made for you to roll down outright not even work. (The big hill incline at one point in Flame Rift Act 1 comes to mind)
Probably the weirdest thing about all this is that the Special Stages don't actually have as many of these issues. They feel more challenging, more compact, and generally just more interesting to go through. Sure, the additional incentive to keep your score up does add to that, but even on their own they generally just seem to have less fat to them. (Though there's definitely still portions where the lack of direction can become a problem, for example the start of Rainstorm Keep)
I think Flame Rift Act 1 is overall the best stage of the main Zones because it feels like the stage's size actually makes more sense due to the heavy usage of momentum and slopes the level has; not to mention the more organic split pathways it feels it has. It's not perfect, but it's definitely the level which feels the most interesting honestly and not boring to outright annoying to get through.
Overall, while I can see the mod has had a lot of talent put into it, I can't help but feel like in the current state it's in, it's just not a very engaging time. Considering it's not done though, I hope the team can see this as a means to improving things going forward.