This is a question I grappled with.
According to Google:
Google Wifi and your privacy - Google Wifi Help
Importantly, the Google Wifi app and your Wifi points do not track the websites you visit or collect the content of any traffic on your network
That being said, it will be up to you if you trust them or not.
A buddy of mine said that when he got it a few years ago, searches he was doing on Google were showing up as ads on his wifes phone. So she figured out that he was taking her to Miami for an anniversary.. I was like YIKES. But he said in the last 6 months or so it hasnt happened. YMMV