A lot of BTC holders has being harassed on the street and even in there homes by criminals,who forced them to transfer all there coins,on this case BTC ATM is a no go area it's very risky.
$5 wrench attacks have been happening a little more often, but we cannot say that it has happened to 'a lot' of bitcoiners, and even if we should protect ourselves from such attacks, but that is not enough reason not to have BTC atm's. Criminals attack people everyday and steal fiat and other valuables from them, it is not just BTC that can be stolen, should we stop fiat atm's because of that.
I am not really sure if we would consider this risky just because people may buy or sell bitcoin at Costco, doesn't feel like it would be anything more than regular ATM as well. Too many people could be attacked based on their ATM as well, it's just that simple, if you are not worried about using normal fiat ATM's, then you could also do that with bitcoin ones as well.
Plus, it is not a security risk for Costco neither, they have security all the time as well, it is not really that complicated for them to just also take a look at the ATM as well, just have an eye on it, like they have eye on every other product they have. They use security cameras as well. So basically both the user and Costco isn't doing something they weren't doing before.