Advice says to either limit use or not to use it at all.
We tried it, and it worked great. We got some speedup due CPU needing to prepare less data, but when doing research how different APIs work with geometry shaders, we found out that Metal (and therefore MoltenVK) on macOS doesn’t support geometry shaders at all. After more digging, we found an article called Why Geometry Shaders Are Slow. So we tested using the geometry shader on a range of PCs in the office, and found that while it was faster on PCs with new graphics cards, the older machines took a noticeable performance hit. Due to the lack of support on macOS and the possible slowdown on slower machines, we decided to drop the idea.
Geometry shaders are inefficient especially when expansion is used like that and that's the case that GPU vendors explicitly recommend against.
I just wanted to let you know that geometry shaders have been virtually abandoned, since with them still being in the API not everyone is aware of that.
GPU Programming Guide - GeForce 8 and 9 Series (Chapter 4):
But in general, the potential for wasted work and performance penalties for using a GS makes it an often unused feature of Shader model 4.
Geometry shaders can be used for Level-of-Detail subdivision of geometry, and people did expect to be able to use them for that. But AFAIK it turned out that their performance in that scenario is horrible, which is why we now have tesselation shaders for that purpose.