I have had nothing but problem using Unity's grass out of the box. I use Vegetation Studio (non-pro) specifically for grass. If you deep profile it, the performance spike from turning on of new terrains with grass added to them is ridiculous, and it's caused by the grass, not the terrains, though that's not something everyone does. I also had problems where grass would not show up in builds back when I was trying to use it through Unity terrains, or show up whitewashed, or pink, etc. That was a long time ago though, I'm not sure if I ever solved it without going 3rd party.