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.
