JANA wrote:
For any type of accuracy, depending on the power grid is not a dependable
option! There can be noise, interference, and power dips. On the short
term, the power grid will be accurate, but over the long term, it can be out
by a fair amount.

All wrong.

Clocks essentially filter out all the trash, the long term accuracy is
guaranteed by the generating companies, and the one thing that gets you
is really power outages only.

But that was teh one thing you don't mention.