If you haven't already, get rid of PS2 Unleashed. I understand it was built on the '06 engine, which should explain to you the level of quality you'll get out of that one. Also, I apologize in advance. You sparked a rant.
Sonic Adventure 2 was the last 3D Sonic game I really enjoyed despite the rage-inducing Knuckles/Rouge stages (although I will say that Colors and Generations were very good, but didn't have the same "feel" I was looking for). Sonic Heroes was the first Sonic game I actually ragequit after being infuriated at the controls and the game's overall lack of polish. Sonic went from a very definitive feeling of acceleration and weight to a system where he accelerates way too quickly and has no weight to his movements at all (turning on a dime completely without slowing down threw off my entire learned knowledge of how Sonic games work), making him way hard to control in comparison to SA2. But the Special Stages were the last straw. I actually got to one particular Special Stage where the tube was making a right turn while going downhill, and the line of balloons were on the inside of that turn. There was something about the geometry of that inner bend that actually somehow managed to slow down my forward momentum, screwing me over on the time limit, effectively punishing me for playing the Stage how the Stage was telling me to play it. I missed an Emerald through absolutely no fault of my own and quit the game forever. I have not played it since launch month.
Shadow the Hedgehog was an additional kick in the teeth. Based on the Heroes engine that I already didn't like, the story was nonsensical and felt like a terrible fanfiction story given life. If you want to make an "edgy" game with a plethora of guns and a cocky, stereotypically "cool badass" character then your point of comparison should be Devil May Cry, not Sonic the Hedgehog. The guns somehow made the game even more complicated than the mission-based system already made it (with a dumb, non-responsive "YOU SHOULD ONLY BE LOOKING FORWARD EVEN IF YOU HAVE TO BACKTRACK" camera making things worse), and being required to beat the game a bajillion different times to get the "true" ending only added insult to insult. What a piece of shit.
Sonic '06 was released in an unfinished state. It's beyond salvation. I strongly recommend watching the Game Grumps' playthrough of the game for a pretty accurate representation of what it's like to play through that game.
Sonic Unleashed... I feel like I'm the only one in the world who actually preferred the Werehog stages over the speed stages in Sonic Unleashed. Playing as the Werehog, I could at least make-believe that I was playing as Knuckles which made it actually fun (although that being said, it WAS a Werehog stage that made me ragequit the game). I felt like Sega actually lost their way even more with Unleashed. The good Sonic stages in SA1 and 2 combined long running segments with a decent mix of platforming and branching paths to keep things interesting. Unleashed's speed segments in comparison felt slippery, and entirely too focused on holding down the "Blast" button for as long as you could down extremely long straightaways. I hated the awkward transitions to 2D during these stages as well, feeling like in order to do well at those sections I was supposed to already know what was coming right away, as a fuckup in those segments usually meant you missed a jump off of a rail that you didn't know was ending.
In short, I feel like Unleashed sacrificed the tight controls and high speeds of the first two Adventure games in exchange for spectacle (LOOK HOW FAST YOU'RE GOING WOW KEEP PRESSING THAT SQUARE BUTTON GOOD JOB DO YOU GUYS LIKE OUR GAMES AGAIN YET).
Generations seemed like a huge apology letter, and Sonic 4 Episode 2 showed that they were actively listening to community feedback after the bad gamefeel of Episode 1. So if Sega keeps taking fan feedback into consideration like they seem to have been in recent years, then I feel like things will get better. The last thing I want is for Sonic to suffer the same fate as Mega Man.
