The levels aren't poorly designed. There exactly the type of levels they should have been striving for for the longest time and I think it shows with how many people actively like Generations and have been screaming out for a sequel to it.
Granted they're not as good as Yasuhara's but than his level designs would run circles against any other platformer.
That's the problem. Ever since SA1 Iizuka and friends have been intent on delivering a "fast" experience at any cost. Back then it was by putting automated dashpads on just about everything. Now-a-days it's done by offering extremely restrictive level designs. Instead of being a platformer, the game is basically a QTE. I'm not saying it plays itself, but when you're given a tiny hallway to boost through with nary an obstacle in sight for about 30 seconds, the developer's intent is clear: to deliver a shallow sense of speed and adrenaline at the cost of player interactivity. Coupled with an over abundance of homing attack chains and bottomless pits (WHY ARE THESE STILL HERE IN FORCE?) and you've got something that will appeal to hard score speed junkies and not much else. Good physics and any semblance of exploration have been thrown out completely with the bathwater. How Sonic moves through space makes close to no sense but they cover it up with boosts and homing attacks. I can shout "speed should be earned" till I'm blue(ha!) in the face but people will continue to claim that Gens was a return to form even though it clearly isn't.
Anyway, I've made it fairly clear by this point where I stand so I'm going to stop beating a dead horse. You love it, I don't; simple as that. I'm trying to hold the series to a much higher standard than what Gens offers.