This is a popular thing to say, but is an oversimplification...

Call it anec-data but all my banking apps work in GrapheneOS, and I have several installed. There is one that reduces functionality if SafetyNet fails (have to do the 2fa flow every time I restart the app, can't set as a trusted device and notifications don't work) but it still works to access my account.

That said... I haven't tried to use NFC payments and do carry around a secondary iPhone 15 as my "business phone" these days that pretty much just has payment/banking apps on it, just in case one bank or another decides to suddenly nuke their app on my main phone...

Good riddance, no more spying, no more ads in notifications (in my country you can use banks via browser. Also, instant transfers by phone number are free).

Not necessarily, I have quite a few that work.

It's crowdsourced and therefore incomplete but https://plexus.techlore.tech/ has reports of compatability with the complete absence of Google Services or a replacement like MicroG.

Here in Switzerland my experience is that the big banks like UBS and the cantonal banks tend to work, while the smaller things like McDonald's and my credit card providers tend to break because they have nonsense Play Integrity requirements.

I haven't come across a banking app in the UK that doesn't work with GrapheneOS. HSBC insists you use the AOSP or Google keyboards but otherwise no issues.

There are different levels of anti-user checks. Some only detect unlocked bootloader and/or root. Others use the play integrity anti-feature provided by Google. GrapheneOS tells you when apps request play integrity checks, and you'll see that a lot of apps do these requests constantly, even if they don't actually block you for using an unlocked or non-vendor system (custom key but otherwise locked and not rooted like GOS).

We really need a more foolproof technical solution for this if general purpose computing on the mobile phone is to be preserved. Perhaps some type of a remote control scheme to operate on a "slave" device. Failing that, if I do need one of such apps needing "strong" integrity, I'd probably look into getting an iPhone for those.

I use GraphaneOS and have had zero issues with the ~10 bank/brokerage apps I use.

The experience varies by country, here in Finland I haven't had a single banking app complain about an unlocked bootloader or a custom OS.

Every banking app works perfectly for me on GrapheneOS.