It's not just the green skin.
Look at any movie made in the past several years which relies heavily on CGI for stunt work. It looks fake. From pod racer crashes in SWTPM to the animals in Gladiator, you can tell it's not real.
As for skin being hard to render realistically, they seemed to have done a very good job in T2, and that was 12 years ago. The effects from Darby O'Gill are better than most of what Hollywood's putting out today.