Well after fixing the issue I had with exporting geometry nodes, I have now been playing with them and researching them. So, first here is an example, exported with @JCPalmer 's babylon exporter :
The method I used to use was particle systems (hair option) and there would be one particle system for each rock/tree type, now, with Blender 2.92 + i just need to create one geometry node (Image 1 and Image2). My geometry node system, here, is built on the terrain and essentially has 3 layers. (Image 2). The nodes in the coloured boxes are essentially the same - just one additional node (Transform Node) for the rocks to lower them into the ground a little. The layers are easy to duplicate and modify as you wish.
When I have the layout I like, then just Ctrl + A in the 3d view window and chose the “Instances to Meshes” and then export. The variation between the three different coloured frames (layers) are just in the density, which object to use and vertex group to assign the object to.
If you decide not to use a layer, you just break the link from the last node in the coloured frame and the “Join Geometry” node - no need to delete anything.
Now people are calling the geometry nodes the “Everything Nodes” as they are not just about spreading objects (instances) over terrains. Image 3 is using one simple cube to create a building with windows on all sides - it uses a Boolean Node, and Image 4 is from Blender 2.93 alpha - using a Google texture map to apply tree instances to park areas on the image.
Not sure I will ever be able to keep up with Blender developments! But I will post more as I explore the new additions to come for geometry nodes.
Now a simple question, you can see that there are 7 drawcalls for the scene, but there are four textures colouring the terrain which uses a script outside the basic Babylon.js. There are 2 materials for each type of tree, one for the rock, one for the sky and one for an invisible wall that stops you falling off the terrain - so seven. I assume that the terrain material(s) are not being counted? Any answers to soothe my weary brow 
I do like that little script.
Stay Safe All, gryff 
Image 1: 3d view and node window:
Image 2: the three nodes:
Image 3: A House:
Image 4: Adding trees (yellow) using a texture :
