There are many schools of thought around this. My sports chiro pushes against concrete surfaces. Grass for me also means a twisted ankle, so it's a tradeoff, but we are made to run on grass and uneven surfaces. One argument is that the more we do it the better we are at it. I'm no expert, but I do know that we need to find what works for us. I run the concrete, but I also run rocky trails because they're hard and they force me to use my innate balance. The older I get the more I need to do just that.