The problem with all CG sets is that the actors have nothing to react to, and it definitely shows at times, as in Lucas' awful second trilogy.
What seems to be better is CG
extended sets, where the CG takes the place of a matte painting. The actors are indeed on a real set, but a short one. The rest of the giant hangar/hall/palace/street is CG. They do, however, have nearby objects to react to.
For example, here's a CG extended set for Galactica. The foreground objects are all real, from the fullscale shuttle to the tools and walls. Beyond one of the arches is a greenscreen, and the set is digitally extended to make it look like it goes on and and on and on. The actors have real objects to relate to.
Same set. You can see the greenscreen beyond the second arch.
And the difference in Episode III. They're standing on nothing, with nothing in the background. Nothing to react to, no sightlines. It showed.