The advantage of using GA is that you can use any of the GA features such as real-time view, analyzing the data for things like mobile opens versus desktop, time of day, etc - all without having to write any of that code yourself.
It will still depend on the client opening the email and downloading images. Using GA doesn't change that.
As for their data policy:
"You will not upload any data that allows Google to personally identify an individual (such as certain names, social security numbers, email addresses, or any similar data), or data that permanently identifies a particular device (such as a mobile phone’s unique device identifier if such an identifier cannot be reset), even in hashed form."
https://developers.google.com/analytics/devguides/collection/protocol/policy
An opaque ID generated by your site (integer, guid, etc) is not PII.
That is my personal interpretation of the policy.
I don't send emails through our SMTP server, though. Our email provider handle that and use their open/click tracking - so GA is something I've looked at but not used.