Man, having to depend on AI, and the way team ultimates work, makes the singleplayer story, on anything higher than normal, a pain. You'll constantly be giving items to them, but they'll barely offer them to you. If you're in the lead, (and 90% of the time you will be if you're at all competent) they'll barely be able to use the tailing boost mechanic. If you're behind them, their lines suck and can make it more difficult to get a boost off them. And team ultimates are the worst. Since they're dependent on the whole team using the teamwork mechanics, you'll be lucky to get more than one in a race. And they are effectively a game of chicken to see who can activate their ultimate the latest in the race. It's like having a blue shell, except there's no disadvantage to holding it for as long as you want. Except your AI team mates are liable to activate theirs early, if they get too impatient for you to activate yours.
And I swear, despite using a speed type character with a kart customized to have near-max top speed, all the other racers don't seem at all slower than me, even on straight sections.