If your site is flagged as secure i.e. You type in mybank.com and it doesn’t get altered, and there is no security warning from the browser then I think you’re ok in public WiFi. If those conditions are met then you are securely connected to your bank and your data can’t be intercepted.
The owner of the WiFi network could work out what bank you’re with, but meh. Or, in my case, they could maybe work out who my employer is.
But for general use – I don’t use one. You could use it to mask where you browse and what sites you visit, but the VPN provider would know, and I’m not sure I trust any of them any more than I trust Google &co.