Likewise with hair, I am one of the first among my peers to experiment in using xGen to create environments so there was a lot of research and troubleshooting going on. The videos don't help. They show everything working perfectly fine but unfortunately my experience hasn't been as smooth.
Save your archives in your project folder. You don't want everything to go to your Documents folder. It keeps your project organized so you know where things go, particularly if you need to archive projects
Make sure your objects are at the center origin before converting them to archives. If you don't when you create the points to generate locations for your archives will act as the origin and your archives will be offset.
Your objects needs to have materials on them prior to converting them to archives. You can not assign materials to the archives.
As materials cannot be assigned to archives, material overrides do not work. This was a large issue due to the fact that I prefer to create my volumetric light fog/god rays on a separate render layer and having all the objects to render with a black surface shader.
If you want your archives to grow vertically out of the ground, such as trees, instead of based on the normals, which can cause issues if your ground is sculpted with dips and hills, you want to align your archive to the world normals instead. Replace the expressions in the TiltU, TiltV, and TiltN with the respective alignU([0,1,0]), alignV([0,1,0]), and alignN([0,1,0]).
For the life of me I could not create grass on my ground plane through using groomable splines or splines with "randomly generated across the surface". Maya will just freeze up entirely. It shouldn't be a computer issue as I have tried on multiple computers and they are high end Dells with 64gb of RAM and nVidia Quadro k4200 graphic cards. I have tried both a singular large plane, slightly sculpted to have dips and hills, and have also tried it on a large cylindrical disk for a turntable. They were sized in relation and scene necessity to the real world scale characters, a tiger for the plane and a dinosaur for the turntable. Grass on neither of the geometry would work The only solution that I could come up with to create the grass was to use splines with "at points you specify" and I had to manually paint in the points where I wanted the grass to grow. Not optimal and very time consuming and I found the result to be less the desirable as it doesn't look very lush and there is no density control.
There may have been an issue with the network or it may just really be due to all the leaves and VRay having issues rendering transparencies but it took me 6 hours to render a single frame.