some czech user reported a broken database when he used a repair point on Microsoft Windows.
The reply from Microsoft was, so it is not a Microsoft issue, but Postgres issue, because Postgres doesn't handle VSS process correctly, and then restore point can restore some parts of Postgres's data too.
That is pretty much trying to do file system level backup [1], and the same caveats apply. Most likely, anything but a repair point with a shutdown server will result in a broken database when restored.
In order to get a consistent copy there has to be a specific Postgres Writer [2]. There are some samples on how to use the interface with the Volume Shadow Copy Service [3], but I do not think there is an interface to a file-system consistent snapshot in any other system.