Here is the thing. Most of the time, guys who talk about training in the grass are doing so as an alternative to “sport”training. It’s generally what follows a statement like, “so, are you ever going to have wet grass in the cage?”
My belief is that without some kind of application, training in the grass won’t be particularly helpful. In other words, if you don’t actuall fight, you won’t really be at a point to understand fighting on a wet surface.
If you aren’t a cop, bouncer, or other professionally violent person, and don’t compete, chances are good you have more fundamental gaps in your skills than fighting on wet grass.