Google's Project Fi experimental carrier is improving its network coverage. Whereas up until this point Fi acted as a MVNO (mobile virtual network operator) using Sprint's and T-Mobile's infrastructure, now it has added US Cellular into the mix.
Thus you should get better coverage where US Cellular is available, which right now means 23 states. Over there US Cellular apparently offers 4G LTE coverage for 99% of its subscribers, both urban and rural.
As before, Project Fi will always choose to use the network that has the best coverage where you happen to be at any given moment. Google says Fi customers spend about 95% of the time connected to an LTE network, and they have a connection 99% of the time thanks to the usage of multiple networks. This should improve further with the support for US Cellular. Speaking of which, this will be rolled out to all Fi users "over the coming weeks".