Re: NS&I too
Even their web site is poorly implemented.
The login requires an extra ID (NS&I number) in addition to the username and password, so using a built in password manager doesn't work (by default). Not that much of an issue, as I store the details in a separate KeePass file anyway, but still.
But worse the initial login always (for me anyway) just drops you back to the login page, no error, no indication of why, and you have to put the details in again!? 2nd time it will get in fine.
They do not have the option of any sort of proper MFA (e.g. code generator etc), and as far as I can see, they force the use of SMS for one time access codes, which are used if you log on from a 'new device'.
And I say 'new device'. because on first access from an actual new device, it goes through the SMS process to add it as a new trusted device, but this doesn't really work properly. Not sure how they are fingerprinting the device/browser, but it only works for a short while, like days, after which you have to go through the SMS process again, even though I'm on the same machine using the same browser.
Even their secure messaging system is unreliable. first few unread messages you read, remove the 'unread' status, but after that, they stay at unread, even after being read! I have to log out and back in and read the messages again for the status to be updated.
And don't hit page reload or the back button at any point, as they'll just log you out!