Safe is a relative term. Banking should be GENERALLY safe on a current and updated device whether iOS or Android. Anything can be hacked, but your chances go up if you use older and unmaintained software.
Most respected security experts generally seem to think that iPhones with their hardware and software security solutions are the best choice for most people. They have one narrow funnel of hardware and a single software solution so it's easier for them to control things. Android is a much more fragmented ecosystem and manufacturers use a wide variety of hardware, and implement the software in various ways...not all of those are equal.
Most custom Android ROMS are actually LESS secure than a normal, updated Android installation due to the fact they are working with older bits and pieces in the software as the manfacturers are no longer updating the device's firmware and drivers. They can be more PRIVATE by giving you more feature control and removing Google's spyware etc...but they aren't more SECURE generally. An exception is Graphene OS, which is a custom Android ROM created specifically with security in mind and only works on Pixel phones that are still receiving manufacturer support.
Thus my comment about eye safe and "as secure as possible" not being available in a phone right now. The secure phones seem to bother most of us. Personally I do banking on a computer and never on a phone. I probably wouldn't bank on a phone even if I had one of the above. I cannot say it's terribly dangerous to do it on a OP3 with Paranoid, you might never have a problem, only saying I don't feel comfortable with banking that way as it's added risk I don't see a reason to take.