Actually, the AI in MKWii has some of the least rubber-banding the series has seen. As a result, I've found it to be one of the easiest games to earn all the golds. Overall though, the series has always had poor AI. I don't care though, multiplayer is where it's at.
I also enjoyed the battle mode in it, I like team games and playing 6-on-6 online was the only real time I've enjoyed a battle mode in an MK game. The arenas were fun too.
I don't know if people are considering all factors when they say that Mario Kart Wii has poor item balance. I actually found it to be the best the series has seen, outside of the first game which hardly used items. For starters, in games like Mario Kart 64 and especially Mario Kart 8, all items can appear in all places except for first, which means that powerful items like lightning and stars can show up for second and third place, and junk like green shells and coins can show up in 12th place in MK8. That's pretty whack. The character-specific items in Double Dash were also pretty unbalanced.
What makes MKWii more balanced is that, while you're likely to be smacked around by items, you're also likely to receive good items in return and regain ground. Overall, I find this tends to even things out, and smart use of items can still be part of the strategy. In MK8, a single item hit can be devastating, though you're likely to be hit by more than one at a time since the game has less defensive options with its one-item limit. The reason is because when you get smacked back by items, you're unlikely to receive enough helpful items to regain your ground, and speed-boosters like mushrooms and stars got nerfed to the point of being nearly useless. This means the main factor in catching up revolves around your opponents getting smacked by items also, which in turn makes the game more luck-based. With less items flying around however, it gives the illusion of being less about luck than it really is. This also makes it much easier for one person to front-run in MK8, since anyone who can escape from the pack is unlikely to be challenged by the pack behind them who are battling it out, with nothing slowing down first place. 1st is often decided at the first set of item boxes.
MK Deluxe may fix some of the balance issues with its two-item system. Should be interesting.
That said, balance isn't really a thing that can exist in MK games without nerfing items to the point where they are mostly visual effects. And at that point, it'd just be a boring ol' racing game. There will always be a luck factor, and I'm okay with that. I love the challenge and the variety that items bring, so a game with less emphasis on them is less interesting to me. Mario Kart Wii is controlled chaos at its finest, and I love it.