I am not going to try to argue with this on the latter point. I have semi-studied the level design principles behind various games, but not for Sonic, which you have. I also only own S3&K and the SCD demo, so my experience with actual gameplay is limited. I am speaking only in terms of my experience when playing the level.
First, it was just way too empty visually. Most of the time all I could see was the platform I was standing on and the background, which was far away and unobtrusive. The walls were not the most graphically detailed walls in the world, and particularly when running along a colorful pipe with nothing else around me, there was really nothing much to look at and appreciate. The technological waterfall like things were a nice and pretty touch, but they weren't (thankfully) everywhere and most of the level just didn't have that appeal.
Second, it felt way too much like a "run right" design. On occasion there was something interfering with constant movement, either a pinball area or some rising/falling blocks near the end. Neither of these I minded. The blocks, particularly as a means of getting to a higher floor, worked well. The pinball areas could have had more interesting visuals but the design was decent. However, the level seemed to rely entirely on those to garner interest, and the rest of the layout was running along flat platforms.
Third, the badniks... , there is nothing good I can say about the badniks, though I found the flying fusion guy amusing. If they had attack strategies I could not notice them. There were maybe four or five in the whole level just wandering back and forth, posing no threat at all.
Fourth, there was very little room for exploration. The paths are really clearly marked, with an occasional possible deviation via diagonal spring or something. I found a shield once, a few ring boxes; but nothing really very interesting. Again, the level layout was hideously undercomplicated. It felt like an engine test with added graphics.
I am not an expert on official level designs, and while I recognize the general Angel Island style in that video, I do not know beyond that. Are you saying that the level layout comes from, at least partially, the original game?
If so, I agree that that would be not be a fair comparison in terms of work invested in making the game, but I was talking only in terms of interest value and enjoyability, and the design source doesn't really come into play there. I would have the same experience playing Sonic Nexus' released level if the design came from Sonic Advance, for example. Yes, S&K A benefits from all the preexisting stuff of Angel Island - collapsing bridges, waterfalls, bouncing platforms, water, scenery, etc. - but you are welcome to achieve the same amount of interesting stuff on your own!