Waitable-timer resolution is the same as the system timer resolution.
Do the he said what??
A waitable timer as as accurate as any other event, which is nearly down to the microsecond.Though of course no guarantees.
Plus, a waitable timer will cause a waiting thread to be scheduled over its siblings by giving it a priority boost. Sleep simply makes a thread "ready" after the rounded-up time is over. Which means it may get to run some time later. On non-server editions of Windows, this usually means 2 quantums, since that is the default scheduler unit.
A waitable timer makes the waiting thread ready and higher priority at the exact time the time is up. Due to the way the Windows scheduler works (serving top-down by priority), this makes a huge difference.
It still does not guarantee that the thread will run immediately, but it guarantees that it will the the first one in its group of similar-base-priority thread peers at the next opportunity. It also means that it may interrupt a peer thread before the assigned number of quantums are over.