Android phone with EE network on a £10/mo sub works fine for my needs ie. WhatsApp, plenty of data, calls and texts.
Can't comment on iPhone, Apple don't exist in my world. Security-wise I guess the nightmare is getting your phone nicked while it's in an unlocked state, or another thing they do is surreptitiously video you from a distance typing in the passcode then nick it. Banking on the phone sounds iffy too because the 2FA (2 factor authentication) is the phone itself, yeah right.