I'm playing on grind assist 10 out of 100 and while I did get frustrated, I think the challenge itself is fairly consistent. The biggest issue for me was the rail transfer, which I think isn't exactly clear enough on what it wants the player to do and that can be improved by making the objective area a little wider most likely.
Guys, want to go over a rail, you can hold the left-stick forward when you ollie (without grabbing) to disengage the game's air-to-grind adjustments. I'm not trying to gaslight and say it's not a very hard challenge, I failed many, many times, but we have to be careful what feedback is actually given.