Your content gets served on Google.com domain with a giant "back" button at the top that takes users back to Google rather than deeper into your site. It also creates restrictions on what you can publish and how you can monetize.

Here are some recent articles about it:

https://www.theregister.co.uk/2017/05/19/open_source_insider...

https://daringfireball.net/linked/2017/05/20/gilbertson-amp