Let me get this out of the way. I love Sonic CD. I still find it my favorite sonic game and I have just replayed it. However, the game is riddled with flaws. For one, collecting time stones is 100%... useless. All they do is give you no enemies in future zones and happy music. My main gripes with them aren’t that they are useless, that’s fine. However, the way to obtain them is frustrating, boring, and straight up unfun. In order to collect time stones you need to get 50 rings or more and jump into a giant ring at the end of the level. Next, you must destroy UFOs in a special zone which has you set in an arena with obstacles and water. The issue is how darn fast the UFOs move. They often are faster than Sonic and can make really sharp turns, making it impossible to predict without losing the special stage a few times. You have a time limit however, and touching the water in the stage decreases it by 10 seconds. This would be fine, but many UFOs are impossible to destroy without jumping into the water. This is simply way to annoying to play for such a trivial reward.
The other big issue with Sonic CD is the cramped level design. Levels are absolutely filled with things. Spikes, springs, stage gimmicks, enemies are everywhere. While Sonic has always been designed with the idea that you go slow the first time then progressively go faster each okay through as you get more comfortable running around danger, this game takes it way too far in my opinion. This game is a maze. It’s really hard to find ways in which you can really just let loose and run. Wacky workbench in particular is virtually impossible to run through without completing a minimum of 5 times.
Lasty, the boss fights. The boss fights in this game are infamously bad for a reason. The first boss is you fighting an egg man with a machine that has bouncers on its fists. The idea of this boss is that you wait for him to retract one arm as he walks and attack the main body while the other gets into position. But this boss can be cheesed easily by accident or on purpose. What you can do is get a shield from a monitor in the stage and go to the very right of the boss arena. You stand behind eggman and wail on him. The ability to delete essentially all challenge by accident is awful. In fact, I did that my first time on accident as well. I only beat him conventionally the second play though and it was still incredibly easy. The second boss isn’t much better. All you have to do is make your way to the top of the stage while getting around 2 spike traps. You make your way up by using spring switches, which can be a bit finicky, but thats it. Its boring, barely even a boss.