That's up to them. If they use a lot of banking apps or mobile NFC payments often, you can't do that on an Android OS that isn't certified by Google. Some banking apps are fine, some are not. Trade offs exist if you go the Graphene or Calyx route.

This used to not be the case and you could run these third party ROMs without issue, but Google added CTS certifications several years back for financial apps for heightened security.