the benefit of containers is that they are all in the same browser. all your settings, extensions, remembered passwords, etc. apply to all containers and you can more easily reopen a site in a different container.

i don't get the login with google problem. you would be logged in in one container and not in any of the others. unless you open a site in your "google container", nothing will happen. the container where you log in to google should automatically stay logged in anyways (i am not sure, i don't use google login, but it works like that for many other logins, hackernews for example) so you should never be tempted to hit that login with google button. the separate process only helps you if you save the google password in the browser. if you are like me and don't store passwords in the browser then the containers have the advantage that it is easier to check if you are in the right container instead of making sure you are in the right profile, unless you use themes to differentiate your profiles.