If you use Segment's Google Analytics server-side integrations, even if you follow all of their documentation / recommendation, it is impossible to properly track your visitors/sessions in GA from anonymous through identified.
This means that if you use Segment like this, you cannot do very basic things in GA like understanding attribution of your product's sign ups. In GA, a brand new session is created for the identified users, which is not correct.
This problem applies to using Segment on the web with analytics.js when the Google Analytics Destination is set to Cloud Mode, or when using a true server-side Source such as Python/Ruby/Node.