Online banking would be safe if all traffic was encrypted.
However, theoretically you could be tricked into a man-in-the-middle attack on a public network. But this has nothing to do with the network being secured or not.

If your browser traffic is not encrypted, everybody on a secured or not-secured WiFi network could sniff the data you transfer.
If the network is not secured, even people not on the network could read the data you transfer.

Browser transfer is encrypted only if the https:// protocol is used.