I pretty much concur about the Cars set.
I got it just to try and acquire the Cars toybox unlocks.
However, my lack of skill with racing games eventually lead me to what I term a "roadblock mission".
It's a main mission and because I can't beat it, I can't progress in the gameplay and therefore, the rest of the unlocks are off limits.
And yes, I failed with the bike stunt missions too.
I'm now actively avoiding anything that involves vehicles.