The few times I've seen this (Google and Amazon I think), my password manager (Lastpass) has had no problem, but I've run into several sites where simple two input and a button login forms don't autofill correctly. Usually the username field will get cleared when the password gets autofilled, so I have to manually paste the username.
I wish password managers would become popular with non-tech people already. I can't wait for a day where there's just a "Sign in manually" link for the few people that manage to remember their 1200 usernames/passwords. Password managers shouldn't need to rely on autofilling inputs at all.
I'm honestly not sure why the password managers don't offer federated login (with SSO helped out by their browser extension). As a website owner, I'd love to be able to throw a "login with lastpass" link on the sign in page.
OpenID provides federated login and is already widely used. It's not frictionless however and didn't really take off as much as people first expected due to usability problems with login-urls as user ids and anonymity to website owners.
I wish password managers would become popular with non-tech people already. I can't wait for a day where there's just a "Sign in manually" link for the few people that manage to remember their 1200 usernames/passwords. Password managers shouldn't need to rely on autofilling inputs at all.