First of all, activate magnet boost, it says you can do it on any map. What's stopping people from making a custom map where magnet is activated before even starting the race?!
Second of all, Landing boost runs would be capped at 1 second as that is the exact time it takes to get a landing boost to activate.
Wings, fuel boost and coin boost would have exactly the same problem as magnet. Nothing is stopping people from making a custom map with coins and fuels already inside the vehicle to have fuel/coin boost activated prior to starting the race and additionally players can change starting position of the car to have wings activate before even starting the race
I think it’s common knowledge for people not to run those categories on a custom map, but for the sake of people who don’t have common knowledge I’ll add it to the rules.