Too many technical and quality issues
I'm usually of fan of Milestone's bike games, having bought and played every motorcycle racing game they've made. MotoGP 18 is a big dent in an otherwise good track record.
So why don't I like it? At launch the regular PS4 version of MotoGP 18 was marred by frequent frame-rate issues, such as stutters and shorter freezes. This has been somewhat patched but is still a recurring issue 7 months later, it happens a handful of times (at least) in any given race. While these issues have never been severe enough to cause me to actually crash the bike, it is enough to suddenly break my concentration and immersion. This is obviously a big problem in a high-stakes racing game like this, where you have spent a great deal of time in learning the track and in setting-up the bike, and have to fight hard for every position.
The AI is another major issue. I've always considered Milestone's "bike AI" to be better than what you get in your average racing game. It has certainly never been a concern or worry for me before. In MotoGP 18 however, the AI behaviour is not very good for a Milestone game; they constantly run into you like bullets. They will literally attempt to push you from behind if you are "too slow", it's absurd. It's almost like racing online gamers or the cursed "drivatars", it's just as irritating. Their sense of awareness is lower than what I'm accustomed to in a Milestone game.
Another major issue is the balance of the game, in regards to the Career mode. It feels uneven and unbalanced compared to the previous games, it just doesn't feel right or good, it feels odd. The game seems to have a weird version of rubber-banding, of artificially boosting and lowering the AI's performance. In one race I'm keeping up with racer X on the straights, but in the next race he rockets by me as if he's on a higher bike class. At first I thought it was due to slip-streaming, but it doesn't feel like slip-streaming, it shouldn't happen the way it does in this game. Should it be an attempt at implementing slip-streaming, it doesn't feel realistic and organic, it feels weirdly designed.
Lastly, there's no managerial mode of any kind, so you get less game content compared to their previous game MotoGP 17. You get to update and improve your bike (stronger engine, better brakes etc.) but that's all. MotoGP 18 does not match the rich and fun Career mode offered in the Managerial Mode of MotoGP 17 and the "Valentino Rossi mode" in MotoGP 16.
There are some good things about MotoGP 18 though; the new customization mode is very generous as you can alter your character as much and as often as you like, during practice you get to do various tests that will gain you credits to upgrade the bike (a similar system to F1 2018) , and the new physics model is very good and can be really challenging. In fact, the driving physics is the biggest improvement MotoGP 18 has over the most recent MotoGP titles; it feels deeper and more complex in how the bike behaves, in relation to the surfaces and bike settings. It almost feels like a return to their SBK games, when their games were more "hardcore", most notably SBK X.
On the whole though, MotoGP 18 is a major set-back from Milestone's previous MotoGP games. The positives do not over-weigh the negatives. It feels rushed and untested, and doesn't have the quality I'm accustomed to. MotoGP 16 and 17 are much better games than this, significantly better. After completing two seasons of the Career mode in MotoGP 18 I decided to go back to MotoGP 17, and was shocked at how smoothly it ran, 90% of MotoGP 17 is simply better. I do wish it had the physics of MotoGP 18, but I prefer having a game that runs smoothly, has good AI programming, and a generous Career mode.
So if you are looking for your first MotoGP game you should get MotoGP 16 or 17 instead, ... even MotoGP 14 and 15 is better than MotoGP 18 in my opinion.