I am adding the uniformvolume material to my geometry to get simple and quick fog. Works like a charm (the green example at the top). Why I get the enable dispmap warning, I do not know - displacement is not enabled.

Now, if I plug in some more detailed colour information (bottom sample), the fog disappears completely. What am I doing wrong here? Plugging into the colour input is just one example. It seems this material doesn't like to get any inputs. That can't be right?

Uniform fog cannot have varying density, hence ‘uniform.’ To vary the density with noise, use a regular volume primitive and volume shader.

The reason the fog disappeared though, is for a different reason. Connecting inputs to materials is guaranteed to break attribute bindings (density is an attribute). Promote the density input with a float parameter called “density.”

The warning you are seeing is because all those gallery materials are really old and were hastily converted to the new material context. They might have bugs related to channel references or unused parameters.