Is sous vide better for lean meat or fatty meat?
A big problem with traditional fatty meats such as brisket, pork butt, or such is that Sous Vide temperatures don't get the meat hot enough to render the fat. Lean meats are hard to get tender because of the lack of fat, so Sous Vide is better for lean meat.

I would argue that for things like Brisket and Pork Butt that low and slow without using Sous Vide will always give better results.

For steaks, you can sear after Sous Vide, but just trim the excess fat because it will be easy to get the steak too done if you try to let it render all the fat.