The Nest WiFi System's coverage and performance is far superior to the Google WiFi System, but both systems would benefit significantly with the addition of a 3rd Band.
With greater bandwidth demand and limited affordable ISP options - simplifying configuration options may prove beneficial for your less Techy Users.
I guess I would say the Nest WiFi Router has a bit better WiFi radio (AC2200 4x4 compared to the AC1200 2x2 that is in the Google WiFi and Nest WiFi Point units), but it may not translate into better performance, since many clients don't support more than 2x2 (2 streams). In any case, I can shed a little light on the thinking that goes into adding a third band dedicated to the mesh interconnect. The chips that are used to build devices like the Nest WiFi Router and Google WiFi are highly integrated and intended to be used to construct commodity WiFi routers. So, in addition to the processor, they have a handful of Ethernet ports and a WiFi controller that includes one 2.4GHz and one 5GHz interface. So, you can build a mesh solution that shares the 5GHz radio for mesh and client-facing traffic using these chips by building normal hardware and implementing the mesh features in software. But, if you want to build a mesh solution that has a dedicated mesh radio, you would need to add an additional WiFi chip to get the extra 5GHz WiFi radio to use for the mesh interconnect. This would add some software complexity, but would also add cost to the parts and manufacturing aspect of the product. The result would be a noticeably more expensive product. It would perform better, but would be in a different cost bracket than it is today. It also turns out there aren't that many 5GHz channels easily available at the wider bandwidths (without using the more complex DFS rules). There are basically two, so using both of those for a single system would likely result in a greater chance of interference with nearby systems (though that's a bigger issue in something like an apartment building). Now, when WiFi 6E shows up, there will be a lot more channels available, and we may even see some chips explicitly designed to support three-channel mesh solutions. I would not be surprised to see Google introduce a future product using something like that in the future.
There is a way to achieve better performance with the existing Nest WiFi Router and Google WiFi hardware by running Ethernet (or an equivalent like MoCa) between the primary and the secondaries. The result is even better performance than a three-channel system could achieve.
I hope this was at least interesting. But, even if it wasn't, please do send feedback to Google using the "Feedback" feature in the Google Home app on your phone or tablet.