One of the only big downsides I've noticed with GrapheneOS is that several banking apps don't work with it at all thanks to being tied to Google's verification ecosystem. Luckily I have hardware 2FA keys from my bank so I can authenticate using that. It also slightly decreases the suck-factor from whenever the phone decides to fly off down a drain. This may not be the case for you, so do your research on what you nee…
If you are using a rather popular banking app, chances are high that it has been discussed in the GrapheneOS forum.
Anyway, with google play services installed, mine have worked out of the box.