LiPo's Dangerous in what sense ?
The MKR's have a charging circuit built in that prevents overcharge etc.
Almost any board can run from A series batteries but as I said if they are the alkaline type you will probably find yourself changing them quite often unless you start using sleep modes etc etc and you haven't yet mentioned what or how you are using the MKR with in terms of ancillary devices I simply cannot say much more than I have.
Maybe provide a lot more detail ?