I use one as a ground sheet, and it's worked great. I think the foam material they use is waterproof. I haven't used it in super wet conditions, but I have used it on top of muddy ground and wet grass. Never had any issues with water coming through it.
It is non absorbent and water will no penetrate unless there is a tear, which can happen. I have used the hell out of mine and am a big fan. Never had an issue with water.
Me too, I use a matty as my ground cloth, it’s getting pretty beat up but tape fixes that. It can also be used to haul pocket gopher dirt to smooth out your sleeping spot.