It should have stayed in the MUD.
There was supposed to be track deterioration. There is not. Moreover, the grooves that you can see on the track are purely aesthetic. There is no real grooves. So, you can go from the far left to the far right of the track without fearing to fall of your bike. That is one of the most important thing in motocross racing.
Secondly, the career mode is absolutely broken. All of the races need to be unlocked with money. However, you will make less money in a race than it costs to unlock the next one. So, you have to repeat your races twice quite often to do so. Do not forget that you also have to buy sponsors in order to win more cash from each races. You also have skills to buy. So, forget about buying some cool helmets that are available. That's the last thing you will have a chance to buy since this does not affect your driving skills.
Other than that, the career mode feels empty. You do races, but there is no standings, cinematics or anything after them. The list of riders that you can select from is limited and only contains 4 fictional characters. For a game that has a full list of real riders, that could have been nice if we could have select any in the list. The tracks are most of the time completely invented ones. There is the last one, which is the Motocross of Nations which is real, that's it for the career.
If you play with a keypad, every menus are not adapted to it. So, you will see F1, F2, F3 without knowing which buttons it is on your controller. It feels like you are in a 1990 game, where controllers are almost not existing yet on PC.
When racing, as soon as you go out of the cones, you will be put back on the track. However, sometimes you do not want to reset your bike. So, you will lose precious seconds. That is probably for preventing cheating, however, this could be certainly improved.
The are few good things in this game. The soundtrack is great. It contains several good tracks, mostly punk-rock-alternative music. It is nice to race against Ken Roczen, or Tyler Rattray and being able to beat them. The teams are also real ones, so you can be sponsored by Pro Circuit Kawasaki and so on.
Globally, this might be a fair game for beginners, but even then I would recommend you more the MX vs ATV series before MUD.