Harry is used to a world of rules and magic, where there are consequences for actions. The Joker thrives on breaking rules and creating chaos. Harry would find the Joker's lack of regard for human life and his unpredictable nature very difficult to deal with. He would try to impose some order and morality on the Joker, which would lead to intense clashes between them. For example, if the Joker was about to harm an innocent, Harry would use his magic to stop him, and this would enrage the Joker who doesn't like being thwarted.
It depends on your preferences. Some people think it's great for its intense storylines and well-developed characters. Others might not be as impressed.