The second major issue is technical. Building something like Spanner requires a very accurate time source that absolutely will not skew. Ever. This is how Google avoids partitions and essentially breaks CAP theory. Perfect time gives you globally accurate timestamps without exchanging data. Distributed transactions without locks or shared state, just usually benign contention.
They're not that hard to build, just no demand. Possibly some issues with ITAR preventing such accurate clocks from becoming commodity hardware? It could easily lead to extremely accurate IMU's which are definitely limited. Not sure, but that's what I ran into researching fibre optic gyros. Atomic clocks could probably be built on SMT scale for a few cents IMO.
As usual, it seems Google is already doing this and has been for years. We either need to wait for the trickle down that Google thankfully does after about 5 years... Or get some deep pocketed tech behemoth to foot the bill for everyone else