Suppose a church employee embezzles or misuses church funds. The church could fire that person without a problem. But, could they sue the person to get the money back? I don't know, but it appears to me that this could become a problem in at least some cases. If the employee could reference any church rule or policy or even misapplied scripture that implies that his actions were justified, then it might make the courts reluctant to interfere in internal religious matters.
I like to imagine what would happen if a power struggle took place within the Watchtower Society. Suppose one faction managed to seize a sizable portion of the assets. Could one faction sue the other? I think it would be difficult, even if some of the actions were illegal. The courts might tell them to handle the situation themselves. I could see it getting really ugly with all sorts of disfellowshipping, slander and acts of intimidation going on.