Which game are you referring to? Are you referring to the entirety of the Genesis Sonic games? Or are you refering to Mario games? Have you even spent much time with early Sonic?
Almost every game in the 16bit era had some sort of artificial difficulty, so that's forgivable. The bit about Sonic controlling badly is a moot point because I disagree... I never really had a problem getting Sonic to jump on platforms. That's more of a personal problem. Sonic is faster than your average platformer protagonist, and his movements were made to reflect that. If you have problems controlling Sonic there are two other characters you can play as: Knuckles and Tails (at least in 3 & K, which I am mostly referring to because it is my personal favorite), both of which can fly, making platforming that much easier. Or you could just, you know, get better. To put it bluntly, I was playing Sonic 3 at 5 years old and rarely had trouble finishing levels (with the exception of the Lava zone which I never beat when I was a kid). Sonic games are platformers. They are based around precise controls, and even Sonic games give you a lot of leeway when it comes to jumping and whatnot.
The level design in Sonic 3 is simply ahead of its time. There are so many branching paths in every level, and some parts are only accessible to certain characters. Each stage progresses in a way that makes sense, with set pieces that move you between levels (Knuckles causing you to fall into Hydrocity Zone, for example). This had not been done before in any platformer since Sonic 3, and it makes you feel like you're actually in a rich world instead of playing a game with black screens between each level. Also, each stage is very complex and allows the player to freely explore, yet the game never leaves you lost or confused about where to go next. Mario, Megaman, pretty much any platformer before Sonic 3 did not have anything like this; you simply moved level to level completing each stage in a timely fashion. The bosses in each stage are pretty sweet too.
Of course, most of this is based on my opinion, but many happen to agree that Sonic 3 had great level design. The purpose of the game is one part platforming, one part exploration... sort of like what they tried to do with SA 1 & 2. The fact that people can speedrun through the game without getting hit shows that the level design isn't really flawed, it's just made to be difficult for people playing through the first few times. Plaformers are about memorization after all. Mario is the same way.