the A->B link is under DDoS or whatever and delivers packets with 10s latency
the A->C link is faulty and has 50% packet loss
the A->{D,E,F} links are perfectly healthy
node B has one view of A's skew which is pretty bad, node C has a different view which is also pretty bad for different reasons, and nodes D E and F have a totally different view which is basically perfect
you literally cannot "detect skew" in a way that's reliable and actionable
issues are not a function of the node, they're a function of everything between the node and the observer, and are different for each observer
even if clocks were perfectly accurate, there is no such thing as a single consistent time across a distributed system. two events arriving at two nodes at precisely the same moment require some amount of time to be communicated to other nodes in the system, that time is a function of the speed of light, the "light cone" defines a physical limit to the propagation of information
The clock sync is just to keep human-readable logs in order for debugging. It’s ok if it is sometimes out of order, though in practice, it never is.
Node clocks can be plenty reliable, but like any other hardware, sometimes they get defects.