It sounds like they are asking about the controversial subject matter and possibly style of his work. As an answer I would focus more on the subject matter being controversial. Here is a bit on his subject matter....
....Upon entering the exhibition, visitors discover paintings that were first exhibited in the late 1860s and early 1870s, a time when subject matter predominated as the determining factor of a painting's success. The themes Homer explored were drawn from current events and topics of contemporary concern - the treatment of women, children, and African-Americans, the status of the American farm, and the preservation of the wilderness - and remain relevant topics today.
Homer's oil paintings were thought to be controversial for the era. He was considered contemporary for the times. His works were criticized for their loose nature and critics considered his brushstrokes to be too freely applied and much too evocative.
In 1875, Homer furthered his painterly exploration by trying his hand at watercolor. Although the medium of watercolor was becoming increasingly more respected, Homer’s works were often criticized, like his oil paintings, for their looseness and suggestiveness.
