The CPU AI in CTR NF is impossible to play against. Anything less than a perfect run in Story Mode will cause you to lose the race (classic mode). Frankly that saps a lot of the fun out of the game.
I have only played MK 8 a few minutes but I assume it is at least fair.
The AI is not impossible to play against it, the AI is fair and the bare minimum you need to have a competitive race without going multiplayer
The problem with CRT is that you need to know how to drift, how to get speed boosts, how to wisely manage items, how to use the shortcuts. It's a game that cherishes skill rather than luck
Of course, a bit of luck won't be bad, but of you are enough well trained player you won't have any problem to beat a race in story mode, at least playing normal, haven't tested hard yet