You are basically asking if Google or Facebook will ever turn evil and command their IT team to steal your customer's account (i.e. initiate an SSO flow) to steal their money.
That's something, despite unlikely, you don't want to underestimate. The bigger the company offering SSO, the higher the reputation at stake. They don't want to lose their entire business for a bunch of million $ worth.
Let's say that you are relying on a third party platform, rather than hosting your own. While it's absolutely more secure than maintaining your code base (Google has their own IT engineering team always up to date with security), you are still giving the user the option to escrow the keys to their apartment to some other company.
I don't want to discuss about these big companies' trustability, but you may just want to add an MFA handled by your platform, especially for sensitive operations.
E.g. if you login with SSO you can just see your balance, but then you need a TOTP code or a biometric mobile app scan to transfer funds.
Still, I have never seen one very single bank allowing users to log in with SSO.
Some banks, especially in Europe, are considering the option to sign up with eIDAS or equivalent national eIDs, but not to use those as sign in mechanism, because it is handled by external SSO entities.
That means you could create your account, and skip the paperwork/selfie part of KYC
(Know Your Customer
) with the option of an electronic flow. But then the bank will give you your own username, password and OTP token.