Don't forget the billion-plus machines out there running Windows Time Service (which strangely has had zero security issues I can remember, even when running in server mode).
W32Time has different kinds of issues, in my experience. With it, one's problems tend to be that, by design until very recently, it doesn't provide to-the-second accuracy.
* https://blogs.technet.microsoft.com/askds/2007/10/23/high-ac...
* https://greyware.com/software/domaintime/v5/overview/w32time...
* https://technet.microsoft.com/en-gb/windows-server-docs/iden...