what makes the item system suck/the game broken?
Oh, so many things.
First off, the fact that you can hold just one item at a time takes away all the little strategy Mario Kart had before, forcing you to keep the item for protection up to the entire race sometimes, instead of throwing it in search of something better. The game straight up punishes you if you dare launching the item trying to get an opponent behind you, leaving you with your butt exposed, especially if you get the dreadful Coin item.
The Coin item is simply a mistake. In first position, it appears too often (up to three times in a row to me) leaving you completely defenseless and also being useless if you already have the 10 coins. It shouldn't appear when you already have them. Hell, it shouldn't exist. I hope a patch removes it.
So, basically, 99% of the time you're just holding that Banana/Green Shell, and if they're used to protect you from another item, chance is you'll have to wait a quarter of the track to take another item. One-two punches happen often so you really can't do anything about that.
The Blue Shell thing is a little better maybe but the Box item is very rare especially in first position, and even if you manage to get it you'll probably just keep for the entire race waiting for the Blue Shell (I used it against the Shell just once and I already beat all the 50cc, 100cc and 150cc cups with a three-stars ranking).
In the end, it comes off as simply boring when played at a low difficulty since you barely see the other players, and it's broken when it's at a hard difficulty for the reasons I said above.
To be honest, the whole game seems incredibly dumbed down to me. The two Rainbow Roads are by far the easiest, most boring and flat Rainbow Roads of the entire franchise. The item system completely removes all strategy involved before and even all the characters/karts feel almost the same, with a couple of exceptions.