I think I'd rather keep the pavements and the floors, etc. rather than the bare ground. More difficult to bicycle on, too
I did a great deal of research last year, and I couldn't find anything definite negative. What I remember was a few that were concerned about running in them, but then they are not designed for running... Also one that were concerned because of some specific condition, but again nothing to do with normal use. they've gotten a bit of hype, and I would take both the good and the bad with a grain of salt.
They're not the holy grail that cure all ills, but I find them a nice variation and I don't think it's a bad thing that they stretch and exercise leg muscles that don't usually get the exercise.