Their FAQ suggests increasing coverage by having one unit act as the router (doing NAT), and having others act as (bridged) access points:
https://support.google.com/onhub/answer/6275097?vid=1-635755...
https://support.google.com/onhub/answer/6275097?vid=1-635755...
If you're doing the wireless bridge you're trading off a lot of wireless bandwidth (and added noise) just to get data from one Airport to another. Far from an optimal setup.