Nothing is safe. The platforms can atleast do a better job to provide feedback or display on the UI, whether the current environment is safe, for monetary txns etc.
Secureboot [0] creates trust between hardware and software, but here you have to trust the hardware. An interesting question is how can software audit its hardware that is driving it ?
ARM TrustZone [1] is another way to offload all the sensitive computation onto another embedded SoC. But, its hard to know what the application is using. You sort of have to trust the banks to do the best job.
Which is the safe one ? The default ecosystem or the jailbroken one ?
I ask because most of the advanced security measures and hardening that I ever considered doing on an android phone required jailbreaking ...