I honestly think Frostbite is significantly harder than Inferno to platinum, even though I got it before Inferno. Frostbite took me about twice as much time and effort. It's hard to explain why, but I feel like most obstacles on Inferno are extremely gimmicky, so once you find a consistent method, you'll get around them very frequently.
Frostbite on the other hand has a few obstacles that require insane precision (checkpoint 3 is arguably one of the hardest obstacles Redlynx has ever made imo, barring skill games like Ice Climb).
Light City isn't too bad. I found the simple bunny hop about two thirds of the way through the track to be the obstacle that gave me the most trouble. It's quite precise.
Btw, as a frame of reference I use, I'd say once you can zero fault any extreme tracks, you've entered the top echelon of players in Trials. As far as going past that point in skill, I feel like the major ways I've personally improved are in my ability to react to mistakes I make, and salvage attempts. The core mechanics the best players use don't differ very much from players that have just begun to zero fault extreme tracks, as far as I've noticed.