If I can be weirdly specific here, I find fault in the level "Above and Below." Not for anything big, but the smaller details of it, like how slightly higher the second incline underneath the large turbine is that you need to do this weird trickshot in order to get enough height. Just in general I shouldn't be having this much trouble just trying to jump up one ramp, it's difficulty put in the wrong part of the level. The other issue I have is with the ramp you launch off after grabbing the shock absorber and hitting the wall with the time travels; I expect to be building enough speed so I can make the next jump off the following incline, but because I hit that vertical wall my marble starts spinning rapidly in the opposite direction, so (and combined with the shock absorber still in effect) when I try to attempt a jump on the following incline to make the gap the opposite rotation of the marble slows down my momentum and I end up undershooting the gap 50% of the time.
I find a lot of fault in the Expert levels as well. Now a good chunk of them are actually thoughtfully crafted designs, but for certain others that old mentality from the original Platinum starts rearing its head back here.
Orthogonality is the first that I had problems with. From the forked falling path onwards it becomes mere guesswork on how to proceed, which I don't consider to be a legitimate challenge and more to be just frustrating in the wrong places, much like with Above and Below, and even though it tells you what to do you only have so much time to actually react before a wall or platform knocks you the wrong way or off the ledge in the case of the final stretch. Additionally on the subject of that final stretch I've found just trying to land on that platform after jumping off the ramp to be more of a hassle than it should be, and worse yet if you fall off and retry the moving platform to the finish doesn't go back to its starting position immediately like it should after respawning, causing more guesswork that makes it one of the vastly weaker levels in the Expert group.
Polymorphism is another one of those weak levels that has a lot of guesswork to it, and more often than not unless you've played this level countless times before you often get knocked off the moving platforms in ways that you can in no way telegraph. It would be so much more manageable if the moving platforms didn't shift around in a way that they clip into eachother frequently; it's needlessly difficult to stay on the platforms when they keep phasing into one another.
Fist Bump is the next level I find problems with, and not really for the concept, because bumper jumping is a great way to get extra air time and sometimes take shortcuts. My problem here lies with the actual shape and design of the PQ bumper; it's totally rounded out like a dome. Consider for a moment the bumpers from gold, they both have flat surfaces on top, making using them to your advantage more precise. However PQ takes that precision away with how their bumpers are shaped and it, once again, causes more guesswork to be able to progress due to how they can launch you in just about every possible direction and very rarely straight upwards like it should.
Emergency Stopping Only is probably my biggest issue in the Expert campaign by far, primarily because of the insane amount of speed building it commands, now the first ramp wasn't so bad because if you have a hard time brushing through the rotating obstacles, you can take a detour and land off to the side, then after rolling across the décor make an easy jump back down to the ramp on the other side; there's room to get creative here, which I appreciate. But then comes the final ramp which actually makes a 180 into a half a vertical loop, which seems like a cool idea to end the level if it were not for the obscene amount of speed to takes to even get that high up. I legitimately took the speed booster before the incline upwards, went all the way back to the start of the runway, did a SECOND mad dash down it as fast as I possibly could (jumping and camera tilting when possible), and used double the speed booster item to get myself all the way to the edge of the half loop, and I STILL didn't have enough speed to make it; that's ridiculous in of itself.
And finally we have Manic Bounce, the finale of PQ so to speak. Now first and foremost, the presentation of this level is insane and unbelievably outstanding, and when you take that cannon to the final area you really get that genuine gamer feel that you've had a long journey, and you're almost at the end at last, it's an empowering feeling. The only thing hindering this grandiose feeling of a journey coming to the end is... well, the level itself. Every single part of this level is centered around only one thing; the bounce material. Now granted it starts off pretty cool and creative, like that one figure 8 pattern where you're weaving in between the blender shaped rotating objects, but towards the end there's just so many curved, cylindrical bounce platforms that you have to ricochet off of that, once again, again, it becomes pure guess work to make it to the next checkpoint, and while it felt really nice to finally make it to the finish pad, it honestly felt like I got there mostly on dumb luck alone, there was no strategy or any kind of critical thinking used to get to the end, just jump, jump, jump until you miraculously get to the other side. For that final stretch in particular it probably would have actually been a better move to just have that be a long, straight runway to the finish; I mean all of the grand objects, props and scenery are already there (not to mention rolling down a runway with that huge cylinder wrapped around it would make for quite a cool sight), so this could have very well been a simple victory lap to take in before you reach the finish and watch the credits.