This is something that gets mentioned a lot, especially from people who don't use Graphene or other custom ROMs themselves. Personally, I have not had any issues using banking apps on Graphene without Play Services. While this is just anecdotal, even if apps required them, I use a separate "secure profile" where I dump all the fishy, but necessary apps that require Google. I used to run a separate Lineage phone with microG for this purpose, but it became too cumbersome