Fzero X is overall a better experience than Fzero GX, simply because the track design is better, in my opinion. That said, GX isn't a terribly entry into the Fzero series, it's actually quite good. Personally, I haven't found the game too hard, but I've been playing Fzero X constantly since its release.
The ramp up in difficultly is actually quite similar to what happens in Fzero X.
I guess the real "disappointment" with GX is that it's essentially a Fzero X with prettier graphics, slightly worse track design (if X is a 10, GX is a 8, maybe) and you can choose who to unlock rather than doing it by beating circuit difficulty.
Also, I found that the AI is better than FzeroX and X had really good AI. Cars feel alot different from their X versions, so that might be part of what's throwing you off. Lighter cars feel REALLY light and heavier cars really feel the hit in acceleration.
My real complaint about GX is NOT ENOUGH BUTTROCK. CAPT. FALCON'S THEME ISN'T
COCK ROCK???!! NO, instead we get LAME PUNK TRASH SHIT.
Capt. Falcon NEEDS the cockrock. Look at those shining golden nipples of doom.