I can see it being controversial if it's an official portrait. It has too much point of view for the typical official portrait - usually they look like oil-paint-photos. This seems to actually convey a sneering sense of privilege combined with vapidness.
(Note - I am not saying that William sneers, has a sense of privilege, or is vapid. I am not not saying that either. I am just talking about a painting that's also not very good)