I have recently integrated with G Suite to use it as a SAML 2.0 IdP for my application. When configuring the SAML app through the G Suite control panel you are only able to map some basic employee attributes like First Name, Last Name and Primary Email Address.
Previously our application has been used with ADFS which is also configured to send an immutable ID. The intention of this is if someone gets married or otherwise changes their name, their email address will likely also change. By matching on the immutable ID we are able to update our record on our application with the new details.
However, if someone was to change their email + name in Google we are unable to determine that this may be an old account by a different name.
I've noted that you can add custom attributes to Users and can probably map these through but we wouldn't want to impose this on consumers of our SaaS. I see that there is an Employee Id attribute but this doesn't appear to be selectable on the SAML mapping screen.