To be precise on this: Redfin covers 75+ metro markets and the vast majority of the United States by population. "Spotty" is fairly inaccurate if your intent is to describe the % of the population that can use Redfin to find a home.
I know that vast majority by population will cover a lot of where people are buying in the US. But when you're looking at areas outside of those population centers, then sparse could apply
Sparse coverage in sparsly populated locations.
For example, Redfin does not cover my home town, nor any of the Eastern CT cities I put in the search box, so for my parents selling their house, or anyone looking to buy in that area, it's not useful.
(I work at Redfin).