Autofill matching breaks in many ways on the same website, so you have to keep on doing it manually. Ex: Chase has about 5 different ways / pages you can enter your login credentials.
Obviously autofill itself can break on complex page layouts, and that's fine. The security comes from the password manager doing domain matching and offering to fill the password when you click on its addon menu.
If they had 5 different ways, that'd be one thing. Lately, I've been seeing different domains. For example the marketing department registers a domain such as AcmeExclusives.com.