Filesystem backup w/R1Soft CDP - will it work?

I'm looking at using a backup system from R1Soft called CDP, and based
on what I've read if I backup/restore the filesystem postgres is on,
I'll get a full backup/restore capability just using files and not
requiring any other tools (not even pg_dump).

I'm just wondering if there's likely to be any data loss if this is
all I do, or if I also need to do some extra database commands to
flush things to disk as part of a filesystem backup.

Any help and suggestions appreciated!