Each balloon provides coverage to a 40km diameter area. That's a lot of balloons to cover rural Africa. It would help a great deal if the balloons could be made to stay in precise locations, like geostationary satellites. I thought maybe they had solved that problem (which would be amazing). But, I guess not. They said in the video that the balloons would follow the wind all the way around the Earth. Meaning, at any given time, the overwhelming majority of balloons are over the ocean. The difficulty, then, is not merely putting enough balloons up to cover the ground in 40km increments, but covering the ocean as well, in a solid band around the entire Earth. It's not hard to imagine that a stupendous number of balloons would be required to provide any meaningful level of coverage.
I think this is more Google hubris than technical innovation.