Why do you use the term "simpler" and what do you mean by "simpler"?
I guess primitive shaders might be 'simpler' in the sense that both it and mesh shaders share a common subset in terms of functionality but other than that primitive shaders has the exclusive capability to map both the traditional geometry and the next generation geometry pipelines to it so there might arguably less hardware implementation redundancies on AMD HW. On NV HW, you straight up have two different geometry pipelines implemented in hardware ...