While SailfishOS seems neat, it’s worth noting that some components, like some drivers, the homescreen UI, the compositor, some QML components and the Android compatibility layer are closed source. The rest of the software stack seems to be based on open source components from desktop Linux. The package manager uses RPM.
Right now I can access my Rabobank account and all its settings through their website. To log in I insert my debit card into a little device called a “Rabo Reader”, enter my pin and scan a “color code” (QR code, but with coloured dots) on the website. The Rabo Reader then generates and displays a one-time code I can use to log in.
IIRC ING texts the one-time code to your phone.
When doing an iDeal payment, you are usually redirected to a page with a QR code you can scan with your banking app. However, there is also an option on the page to select your bank and authorize the payment through your bank’s website. I wouldn’t be surprised if the latter option is phased out when iDeal merges with/turns into Wero, though.
For the foreseeable future all in-store payments using debit cards are going through Mastercard or Visa, both American companies.
I don’t disagree and I definitely trust a Finnish company founded by former Nokia engineers a lot more than Google or Apple. But I think there is a lot of overlap between both the “buy European” movement and Fediverse users and those who prefer open solutions wherever possible. A lot of those people would prefer a solution where the userland and/or kernel and/or SDK is completely open source. They will have to weigh their options based on the fact that while SailfishOS is more open than Android or iOS, it is not fully open source.
By the way, it is also worth noting that unlike Google, so far Jolla has been moving in the direction of open sourcing more components of their OS. No one can predict the future, though, and some people would prefer to avoid any possible future vendor lock-in.