Hard to store, basically. You either have to compress it and store it in a fairly heavy tank, or liquefy it, which means you have a thermos bottle of very cold liquid which can't be stored indefinitely. Nevertheless, there are a number of fleet vehicles that have been converted to NG.
I have a (stationary) NG fueled standby generator. While it's hard to handle, engines love it as a fuel. When I change the oil on the generator, the old oil looks -- after a year -- almost as clean as the new oil.
Urban buses and some short haul trucks use CNG quite a lot. MARTA in Atlanta certainly does. Fueling is a big deal, but it’s not too bad to set it up for a bus fleet covering a limited geography