Nothing proprietary in Spanner but I believe no other vendor has an atomic clock similar to what Google has. Hence they are not able to implement the paxos global transaction lock which requires that all servers participating in the transaction be perfectly synchronized. This is what one of the comments above refers to I think.
So while the spanner paper is open to be implemented by any vendor, they don't have the proprietary advantage that Google has - the atomic clock. So Yugabyte, CockroachDB don't rely on atomic clocks. I tried to get to the ground level basics of this, but I haven't understood this matter completely yet.
I guess using worse clocks would mean using a (slightly?) slower spanner, but I'm not sure what is the impact. In any case, if a big vendor (e.g., Amazon IBM Oracle Dell...) would want something on par with Google's clock they probably can achieve it (though I don't know much about these clocks).
The problem is that the slower it is the more the window for error grows. And the harder it is to recover. It also impose a high boundary to latency. You cannot be faster than your clock error without risks.
Note that even Spanner had multiple downtime due to clock and/or network failures. In these case, any operation lal guarantees are lost. This makes it really dangerous.
