bitcoin banking is probably ponzi. simply because you are hardly earning a decent interest with the blocks halving, really man.
even with the mining, you won't gain a lot of interest in bank; again, halving.
Everything that promises you a certain interest can turn into a ponzi scheme at any time. Even if they had "good" intentions when they started. It's very simple, never let your coins be held by a third party. If that is your basic rule, then you will be just fine.
It is indeed very good. And I think it is also very safe and that is good. But I hope that the banks are bigger and bigger.
So we can use it easy and that Bitcoin will be more used also in the future that will be good for the Bitcoin bank.