Black Knight removes the importance of leveling up Sonic - that's the only real "improvement" over Secret Rings. Instead of being slow and crippled, Sonic starts out fairly average and I don't think leveling up really unlocks new stuff? I never really noticed a dramatic change in Sonic's abilities, anyway.
But everything else is bad.
There was this prevailing idea at Sonic Team that Sonic games needed to have "deep combat". It works like this, basically...
You have enemies with lifebars. What do lifebars mean? Lifebars mean they take multiple hits to defeat. So what is the benefit of enemies taking multiple hits to defeat? Well, you need to have some sort of combat system. Excluding the RPGs, Mario enemies don't have lifebars because all you'd be doing is bouncing on a goomba 3 or 4 times instead of once, and that would be boring and weird.
Sonic Heroes introduced lifebars to Sonic games. Because there are lifebars, Knuckles now has a punch combo you're expected to use to defeat them, in addition to the fairly robust stable of attack options covered by the speed and flight characters and your ability to switch between them at will.
The same goes for Shadow the Hedgehog. Shadow has a punch combo in that game in addition to melee weapons and guns, so all of the enemies have big lifebars that take quite a lot of sustained damage in order to kill (this is mainly because automatic weapons spew out so many bullets very rapidly).
Enemies in Sonic 06 have lifebars, too, but this is where it becomes the biggest problem: Sonic doesn't really have much of a way to deal with enemies with big lifebars outside of spamming the bounce attack, and that's not an obvious solution for most people. Shadow does, however, but his "combat system" consists of mashing the A button. Silver gets off easy: all of the projectiles he throws seem to be more or less instant kills.
Outside of Shadow the Hedgehog (where guns sort of necessitate them), these games having enemy lifebars doesn't really make sense. Combat is neither fun nor deep, it's just something there to waste your time. Instead of pushing the A button once, you push the A button 6 times. Wow, great, woo. So much fun.
Black Knight breaks this out so that instead of pressing the A button once, now you give yourself repetitive stress injury by waggling all the time. But there's still no depth, and if anything it actually hurts the flow of the game. Instead of Secret Rings, where you're constantly moving forward and it's sort of like a basic version of the racing game template modern Sonic games have become, Black Knight makes you stop every 10 feet because 3 waves of monsters sprouted up from underground. Time to waggle them away.
And the bigger, "tougher" enemies just make this even worse. Enemies with shields or the really big guys that grab you if you get in close aren't fun to fight or difficult to defeat, they just take longer. More waggles. More time wasted. No skill involved.
The case could be made that you could just jump over them, but then what are you doing? You're deliberately ignoring a core game mechanic. Like playing Halo without using any grenades, or Diablo 3 by ignoring loot and only picking up gold. You're not supposed to do that, nor is it any fun - but neither is swordfighting, so we're in a lose/lose situation, here.
What is any of it even for? Why would anybody want this?
And the story... is a bad sort of story. Sonic's kind of a jerk, but in the wrong sort of way: he gets summoned in to this storybook world and basically says "Yes, I will murder this dude for you, no questions asked." And then all he does is bitch and moan when they try and teach him how to use a sword so he can actually do the job correctly. And what about Shadow, Blaze, and Knuckles? I get that they're the knights of the round table, but why are they here? They don't have any personality, they don't contribute anything to the story, they just sort of exist so we can have an excuse to fight them, because ever since Sonic Adventure there's always gotta be rival battles, right?
Secret Rings was a breath of fresh air that tried new things and failed at a lot of them. But at least it tried!
Black Knight takes the Secret Rings formula and tries to bring in dated, rotten old Sonic tropes and "normalize" the concept. And it just highlights why those things were bad.
No thank you, sir.
