I've been dailying GrapheneOS for more than 2 years. I haven't had any issues with any of my bank apps. Although there are apparently some that do block usage on non-google-certified OSs. The only limitation in my use cases pre-GrapheneOS that I've found is not being able to make NFC payments through Google Wallet, but I found a bank in my country whose app implemented NFC payments, and made an account with them to be able to convenintly pay with my phone.

edit: and I'd like to add, GrapheneOS brought me back the joy of using my phone. Since 2018 or so I started to dread my phone (and the internet) more and more. Installing GrapheneOS brought back the joy on using these marvelous computers (and self-hosting brought back the joy of using the internet)

There are essentially two separate issues here.

The first is the anti-trust angle. Some subset of bank apps don't work because of attestation and that's a significant barrier to adoption for switching to competitors, so it ought to be an anti-trust violation for the platform to do that.

The second is, you try it and discover that your bank doesn't work. If you want it bad enough you can switch banks, and the fact that it doesn't work is a signal that your bank has a weak security team who is just cargo culting deleterious vendor nonsense without evaluating whether it has any real security value.

(The use case for attestation is completely orthogonal to bank apps because it can't prevent credential stealing from compromised phones running a fake app since the fake app won't require attestation, and it can't prevent attackers from using stolen credentials to transfer funds because once they have the credentials they can just use a normal phone, and that's the case even if the attestation was completely airtight, which it isn't. Meanwhile the devices that can pass attestation are generally more vulnerable because it implies they're running the more-likely-to-be-outdated OS that came with the device rather than a third party upgrade with more recent patches, so they're essentially encouraging their customers to not upgrade their OS. Banks that do this are wearing clown makeup and you have to ask if you trust them with your money.)

The replies whenever this question is asked remind me of when I used to have a Windows Phone and was trying to convince myself it was the best phone on the market. Yeah, I couldn't use YouTube because Google blocked Microsoft's YouTube app; and I wasn't able to deposit my paychecks because my bank didn't have a Windows Phone app or mobile deposit support on their website; and sure, using Messenger or Snapchat (which was exploding at the time) meant using somebody's reverse-engineered WP version of those apps...

But look at all the information I can get from the Live Tiles! Oh and isn't Cortana neat! A little more self-flagellation for the penitent ones who've traded corporate app stores for daily inconvenience.

Whatever issues I've run into I've been able to work around. I don't use tap-to-pay with the phone at all so that's not an issue. The things I get in return for using GrapheneOS all outweigh any downsides.

My bank detected someone logging in with my password from a GrapheneOS phone and made me change my password and scan my face. That was dumb.

With that out of the way, and the device now seemingly authorized, it still doesn't work, because when I log in, the app restarts. That could be a real compatibility problem.