Personally, I do not use my phone for online banking. I have entered credit/debit card details for some specific apps where I want the convenience of paying through the phone, but that's only a very few.
The issue with Apple is that they expect you to have an Apple account and that requires you to enter your valid credit card details.
Alhough I use Macs for computers, I do not want to do that on an iPhone so I have an Android phone.
If it is not an App that you know. trust and have complete faith in, then my advice would be to treat it as you would a dodgy person you just met in a strange pub.
You can them as much as you want, but you might end up regretting it.