My biggest gripe with this is the second half of Tropical Jungle, where the dash pads make you think you could run on the water but consistently fail to do so. I take it that it doesn't have you follow the ground closely enough to actually start it, but... eh. I ended up having to play it with the specific mod that adds water running, but it does not help at all with the frustration, especially since I'm hunting for the Emeralds this time.
Speaking of, that's another issue: since I'm playing as Adventure Sonic (and later trying Silver and Shadow), I noticed it feels too much like half of these emerald challenge stuff is designed for, say, Knuckles or Tails and not anyone else, even if that ends up wong. And even when that's not the case, it takes damn near forever to find it, cuz the maps are somewhat confusing. I think I'd enjoy it more if... say, it had a faint, looping audio cue to hint you to where it is, or environmental clues near the path to it. But without it, it makes the emeralds a massive chore I don't know if I view as worth the Sol doors. If anything, I'd probably enjoy it more if it was the base game's system instead; find coins, beat level, beat NiGHTS, emerald get. I like that there is uniqueness, but I think this isn't 100% the best idea.
On another note, it's novel that the OG 06 textures are used, and I'm aware that they're also in the 2.1.x build, but due to the limited lighting the base game has, they look pretty damn ugly here, especially since the cartoonish characters, enemies and assets clash too much with the photo-realism. I think, instead of overhauling the latter, you and the OG dev could have made matching textures and decorations that are accurate to 06 but stylized after SRB2.