It's funny, I never tried out the Chao garden in the Adventure games because I never had any idea what I was supposed to do there. I even have a link cable with the GBA game, Sonic Advance. Sonic has an awful track record with the quality of his games. I think I found something wrong with just about every entry of the series. Yes, that includes the originals, the Adventure series, and even Generations.
As far as the originals, I feel like every level that wasn't the first three or so weren't playtested well enough. I commonly found myself stuck in a wall or in the ground just because I was speeding through a level and making creative jumps. Other times, certain hazards are just dickishly placed. You're running at full speed, stop immediately when Sonic hits an object that you should jump over, and spikes come up from the ground to make you lose your rings. It's something the player cannot see even if they were paying close attention.
The Adventure series is pitiful in both level design and control. Honestly, though I rank 06 as the worst, it's just behind SA1. There's very limited camera control, the scripted sequences of running really fast through an area can kill you if you realize that you're not controlling the character, but don't see that control has been given back to you before Sonic runs clear off a cliff or the side of a loop. Sonic Heroes was especially bad about this. I don't think I have to mention the gameplay in Big's story. And somehow the treasure hunting segments with Knuckles were made even worse in SA2. Every enemy is either not worth killing or has a cheap attack that's almost guaranteed to hit you. Nearly every boss in the two games was a joke of a challenge. You know what? Joke is the only word to describe the games. Because '06 was terrible in ways that made the player actively angry. The Adventure games are terrible in ways that are funny or random. Stuff that we can just make fun of. How people suddenly decided they're good games, even the best, is beyond me. The only aspect of those games that impressed me, was the multiplayer I tried out in SA2, after finishing the game. Racing the other player with characters like mecha sonic and chaos is the only entertainment that can be found.
Generations isn't a bad game, not at all. It was just really short. They finally perfected both Modern and Classic Sonic gameplay, but gave us something that the average player will beat in four hours. There's a lot of unlockables and challenges, but I felt myself wanting a lot more. Especially since there's quite a few Sonic games that weren't even represented. I feel like time travel wasn't really taken advantage of story-wise except for a few jokes.
Nonetheless, I have high hopes for Sonic Adventure 3. They got the gameplay down, so I can't see how it can be a COMPLETE failure.