To be quite honest I think people are being pretty hyperbolic about the F2P system. It is extremely generous. For one, the game continually throws premium currency (gems) at you, and you can earn more in any place you can earn regular rewards as well. On top of they, the game throws money at you as well. The first few hours of the game you get more money thrown at you than you can spend. If you're not collecting parts and buying parts outright, that's your fault. If you're wasting gems on anything but respins for quest items, that's also your fault. If your main goal is to pump upgrades into your bike until you get a free-win scenario (some games you can manage this) for the rest of the game, you're willingly pursuing a grind that is not necessary. I don't think it's nearly as grind-heavy or upgrade-heavy as y'all are claiming except for the saloon/garage upgrade stuff. Don't understand bringing up the need for real cash over and over either; few games are as generous when it comes to providing premium currency within the game period. It never makes me feel like I need to buy gems to survive.
That said, it is not meant for long play sessions, no real mobile game is because of the nature of the hardware, and as such, the fuel system isn't that much of a nuisance to me (especially considering for a good chunk of the first few levels, it's impossible to even run out of gas before level king up).
There are only two real difficulty bumps I have encountered thus far, the first was fairly early on when reaching the village; you realize you won't be able to win anything right away after trailblazing through the first few levels. Fitting, because you get introduced to the upgrade system around then. The second one is when you're chasing Butch up into the very northwest stages, where you will need to pump time and gas into upgrades and can't really blitz through using skill because you're outgunned.
Wait 'till y'all get to the player ghost challenges which are seemingly random in difficulty. Now those are roadblocks. Thankfully they're not relevant to the main story/progression.