Before you even insert cash, the machine asks for your QR code. It makes sure it is a valid address before accepting any bills.  As long as your wallet and devices are secure(which is out of BTM's control), you should feel very comfortable using BTMs. They will not let you insert more money than they can cover. I have not used Skyhook, but the major BTM manufacturers have never had transaction issues I am aware of. Bitaccess, Lamassu, Robocoin, Genesis1 are all trustworthy machines.