There's bound to be tradeoffs between scrappy open source communities and trillion dollar industry behemoths. The fact that it's this close of a call is pretty amazing. And really you can blame your bank for not making a usable mobile site. A lot of businesses like to force users into apps because it helps with engagement metrics, not because there's any functional benefit.
Its not even a matter of tradeoffs - banks just suck major ass so, of course, their piece of shit apps are extremely fragile and only work under just the right conditions.
That's not any OS' fault, that's banks fault. That's been my experience with every bank I've used so far and yes - they often break on certified OS' too! I've been on the phone with support!
Because they make bad software, period, and we're all just forced to use their bad software.
Many many people have banking apps that will not work on non-Google-blessed devices, use banks that have mobile websites that are terrible, and need to do mobile check deposits (which is usually only available in the app, and not the mobile website, if the bank even has one). And no, we're not going to "change our bank".
The reality is that there are so many things that break, sometimes in subtle ways, when you try to use an alternative Android OS. Some people may not have any problems, and that's great! But many -- I would dare to say most -- will.
And there's also a ton of uncertainty: I don't really want to wipe my phone, install GrapheneOS, spend hours messing with it and setting it up, only to find that something critical doesn't work, and now I have to flash back to the stock OS, and hope I can restore everything the way it was.