There is a way to lower risk and exposure. You can program the ATM to buy BTC from an exchange as the customer is buying BTC from the machine. So if the fee is 10% and you insert $1000, you get $900 worth of BTC and then the machine can buy $900 BTC from an exchange immediately. I currently don't have it set up like that, still working on the settings. For now, I am doing it manually on large purchases.
If I get more machines, I will keep a fixed amount of BTC in the wallet...say 10 coins. And then I will set it up to have the wallet topped off to exactly 10 coins for every transaction. So the only risk would be those 10 coins going to 0 or being stolen by a hacker.