The major challenge posed by a diverse workforce comes from the same source as the major benefit of such a workforce. A diverse workforce is made up of people from different races, different cultures, and different backgrounds of other sorts. This means that the people in such a workforce can be very different from one another. These differences are a source of benefits, but they can also make it very hard for supervisors.
There are two main reasons for this. First, it will be difficult to deal with all of the different kinds of people without having misunderstandings arise between the supervisor and the employees. People from different backgrounds will have different ways of understanding things and might miscommunicate. It can be hard for supervisors to understand the needs of people from other backgrounds and to know how to motivate those people to do well.
Second, the existence of diversity in the workplace can cause problems between workers that supervisors must mediate or arbitrate. People from different backgrounds may dislike one another on racial/ethnic/religious, or whatever other bases. Even if that does not happen, they may simply misunderstand one another and those misunderstandings may lead to conflict. In these ways, a diverse workplace can lead to problems for supervisors.
The main thing I would expect to find rewarding about this is that any difficult job is rewarding if you do it well. There is a real sense of accomplishment that goes along with doing a hard job well. This sense of accomplishment is much less present if the job was not hard in the first place.