But now there is chrono and I wonder if there is any reason not to use it.
I'm not sure of the status quo, but high-resolution clocks were not used in all standard libraries, making corresponding code non-portable. And the syntax may be slightly more verbose, but of course you can use typedef/using and namespace aliases.
From my amateur perspective Clock seems to be wrapping ctime to get around its ugly API
No,
sf::Clock wraps operating system specific APIs. <ctime> does not provide high resolution clocks. And the API is not ugly, just low-level C.