Following the release of the switch the first game we picked up was Fast RMX only for the fact that it offered 4 player multiplayer. My first impressions of the game were very positive you jump right in with no tutorial. This was fine as it teaches you as you play as you go and the controls are very simple. The game works on unlocks, you start out with 3 different choices of cars and 3 maps to choose from and every time you finish a race for the first time you unlock a new race and car. It forces you to start on the easiest difficulty which is weird design but by the time you get to some of the harder maps even on beginner its quite challenging.
My best comparison is to mario cart, I think of 50cc being a level that all players should start at if they are new to gaming but if you have any prior experience with racing games or even just gaming in general you should be able to consistently be able to make first place. This game doesn't have such a thing, by the time you get to the Indium map set even on novice the difficulty ramps up exponentially and you will find yourself replaying the map set over and over just to reach first place. They add in environmental hazards as well as confusing camera angles and computer players that all seem to have a faster car then you and one small mistake will set you back from ever reaching that first place. This is something I would expect out of advanced or expert difficulty. This game is not for the casual gamer unless you want to stick only to the first three map sets. If that's not challenging enough there is a hero mode where if you run out of boost you lose and you only win if you get first place.
Some of the mechanics are super unpredictable and get to be very frustrating after spending time with the game. When you run into an object or fall off the map and get destroyed sometimes it teleport's you way ahead of where you actually fell which if used correctly you could gain a lot of ground in the race. This is great for single player if you're just trying to get a good time but in multiplayer it could easily change who's winning and who's losing very badly taking fun out of the head to head challenge. The worst mechanic is the interaction with other racers on the track, there must be some way to control this or maybe its just bad design but when you bump into a racer one of you will spin out or your car will shake and you will lose speed. It seems to change when one is boosting they spin out the other opponent which is fine except for the fact that it doesn't do that all the time, its not reliable.
TLDR: Easy to pick up and play for new gamers, after the first three map sets it becomes very difficult on easiest difficulty. Unreliable teleportation and car hitbox's are a huge setback for local multiplayer fun.