To me H3 looked over-engineered and unnecessarily complex. Hexagons don't tile nicely at multiple resolutions, for one! Just overcoming that is decidedly non-trivial.
Implementing Google's S2 is simpler, but it has the same overall benefits as H3 such as a hierarchical data structure.
H3's algorithms involve some intricate maths, but the library itself is conceptually simple. Check this page out for some really fun and neat ideas: https://www.redblobgames.com/grids/hexagons/.
Uber internally had extensive research on what kind of grid system to use. In fact, we started with S2 and geo-hash, but H3 is superior. Long story short, hexagons are like discretized circles, and therefore offer more symmetry than S2 cells[1]. Consequently, hexagons offer more uniform shapes when we compose hierarchical structures. Besides, H3 cells have more consistent sizes in different latitudes, which is very important for uber to compute supply and demand of cars.
[1] One of the complications is that H3 has to have pentagons to tile the entire world, just like a soccer ball. We can easily see why by Euler's characteristic formula.
Implementing Google's S2 is simpler, but it has the same overall benefits as H3 such as a hierarchical data structure.