Simple Bank, when they were a thing, published this "look at how awesome we are" post about moving to Riak from Cassandra. (I think that's what it was; it's been a long time.)

"We're going to be so reliable after this; big banks don't stand a chance" was what I took away from it.

The migration went horribly. Tier 0 clusterfuck. Debit cards were completely unusable and they lost ledger data. That and the occasional duplicate transaction (like the duplicated student loan payment that put me in a really bad spot at the time, which I was only given $50 for the inconvenience) was enough for me to _run_ bank to Chase.

So, yeah, I get why the big banks are totally fine with keeping their mostly-rock-solid core banking functions on the mainframe and stick with datacenters everywhere.

100% why I bank with an old bank that runs mainframes.