This game is an exercise in frustration. I have reached level 24 and still haven't unlocked any of the upgrades or other bikes. I have unlocked one of the racetracks, but can't remember how I did it. Beating the boss maps doesn't unlock anything, so it must have been another way such as one of the loot stashes, which mostly just give you money that I can't spend since nothing is unlocked.
The police chase after you any time you pass them, which you will do even if you hold down the brakes the entire time because they drive so slowly. The chase vehicle on the other hand is very fast and difficult to evade using just the speed boosters and your normal nitro boost. If you are lucky enough to get one of the nitro powerups, you may be able to evade them, but you will run into another police car soon after and the whole process begins again. Even worse, in most maps you are doing this while also trying to complete the objectives for that map.
This game could be fun if there was any sense of progression, but it feels like more of a grind with little reward, so for that reason I can't recommend it.