Impish can be just as good as Careful considering that is the weaker stat of Dragonite's defenses and OU tends to be physically oriented anyway. It might even be better.
Impish / Careful / Bold / Calm are all great natures if you want to make use of Barrier. All that changes is Dragonite's attacking move for my hypothetical set. If physical, you're using Dragon Claw. If special, you're using Dragon Pulse. Special does have its merits because you don't have to worry about contact effects, intimidate, or rocky helmet.
Impish / Careful / Bold / Calm are all great natures if you want to make use of Barrier. All that changes is Dragonite's attacking move for my hypothetical set. If physical, you're using Dragon Claw. If special, you're using Dragon Pulse. Special does have its merits because you don't have to worry about contact effects, intimidate, or rocky helmet.