Sonic CD is good and people misunderstand what makes it good.

Sonic already kind of has a problem where people want an extremely specific form of gameplay out of it and mentally disengage whenever it's not scratching that itch. So it's not really surprising that people dislike CD so much with its 'playground' mentality to level design; aimless and chaotic routes that weave inorganically with each other doesn't facilitate the speedrunning flow that people want from the classic sonic model. But that's kind of the point from an aesthetic sense? I always took the ruptured level design as a complement to the game's narrative themes of untainted nature becoming abruptly industrialized, and that messy stylistic relationship gets reinforced further by both soundtracks: JP bringing out the dreamlike pop that NiGHTS would later champion, while US is more overtly ominous and uncomfortable. The game wants you to feel that environmental conflict between the harmonious chaos of Little Planet's flora while being corrupted by Robotnik's bizarre machines. Even from a gameplay standpoint, there's still a lot of fun to be had in learning how to workshop optimal routes for time trials. There's a reason the stages are so short and the game rewards you so heavily for those time attack scores.

The only major failing that I think makes CD hard to come back to is that for how open-ended its levels are, it doesn't give you a lot of incentive to explore them. The metal Sonic generators and transporters are the only objects that occupy these stages and reward you for trekking around. But I also don't really know what the design solution to this would be: Adding spontaneous collectibles could easily turn exploration into a begrudging chore and take away from the freedom of exploring levels at your own pace. On the flipside, I think the existence of special stages as an alternate means for unlocking the true ending is kind of genius: You can take the exploration route, or stick to the Sonic formula of platforming fast, and this duality supports the franchise's long-running depiction of their character's agency and motivations. Sonic is more than just being the hero; he's doing it his own style and he won't change.

I hope CD being ported again via Origins doesn't introduce another wave of jaded thinkpiecers trying to tout CD as the 'overrated letdown' of the franchise. It deserves more appreciation for the ways Sonic's framework allows for experimental design.