At least some of his unrealized potential has to be put on Cavill himself. For whatever reason his offscreen charm hasn't completely translated to onscreen. Part of it is the material. I assume part of it is him as he hasn't really wowed in his other films. I figure THE MAN FROM UNCLE is the closest thing where his performance was liked.
The biggest defense I have for Cavill is that nobody has really come off loved in the recent films. You have actors far more acclaimed then him, such as Amy Adams, Costner, Crowe, Shannon, and Lane. None of them have been universally well-received. That leads me to believe that the material is the biggest culprit. Even if you have charm, warmth and charisma, you really don't have many opportunities to display it.