Yes, I have a UPC Giga Connect Box. You should set the UPC box to bridge mode, otherwise you will have two routers connected in series. It works, I had to do it once because for some reason a friend's VPN connection to his employer didn't work via Google Wifi and I couldn't figure out why. Sunrise, for example, does not offer bridge mode with your modem, you simply have to deactivate the wifi, otherwise in the worst case the signals will interfere with each other. If it works, bridge mode is the recommended way to attach the Google Wifi.

I don't have UPC but generally it is better to put the router in bridge mode because otherwise you have double NAT. I can't put my Wingo box in bridge mode, but I have 2 internal networks now.

So if it works I would activate bridge mode