Level 10 seems too big of a hurdle, I know it'll show up later in the full game but it seems really unfair to the players including me. I managed to get past it using the geared wheels but it requires too much fighting with the mud so I want to pitch this:
Remake Level 10 to be simpler and perhaps throw in a video tutorial so people know how to build the right car to get through mud and how to fight it as well. I suggest removing the heavy ramps up and down and perhaps making the mud smaller.