Using voice commands to make payments sounds like a bit of a security risk, especially considering Siri doesn’t distinguish in any meaningful way between difference voices. But, you do have to unlock your phone to use the feature (you can access Siri from the lock screen but you can’t confirm a payment or request money without unlocking), so it’s no more unsafe than any other app that needs a PIN or Touch ID access to use.

http://www.theverge.com/2016/11/10/13582630/paypal-send-money-siri