The advantage of also being reachable through IPv6 should be obvious: You can also use bunq if you're on a IPv6-only network. Granted, maybe rare today for regular consumers, but it's becoming more and more important for some set-ups. And with carrier-grade NAT, I personally don't really want to use my IPv4 anymore. I don't want to be associated with the actions of any other users. So it is not something that bunq needs on their end, it's something that users need (and will need more and more in the future) to actually reach bunq.