1Password was one of my test extensions and the one I use every day. It works most of the time because username/password combos are the most common autofill configurations.

So common that most password managers don't consider the case where you want to autofill a password without a matching username field on the page. "Password confirmation" being the classic example.

They just don't handle anything other than the main case very well.

Interesting. The weirdest site I use on a regular basis is Mailgun and it has this scrolling form which isolates entry for email/password/2fa (which, as an aside, I hate) and it gets it right every time. I can't remember any instances of it going wrong, actually.