disk backups

hi,
would someone be so kind as to comment on restoring a database from
file backups? for instance, if i backed up the postgres files, once
the database was down, and there was a subsequent problem, a file lost
or a disk crash maybe, would restoring the disk from the filebackup
cause any consistency problems with postgres?
please assume that all the files on the disk were postgres files
and the backup reflected that.

how about trying to restore just one or two files after an accidental
deletion, maybe? again from the disk file backup instead of from a
pg_dump.

how about using those same file backups to move a database to another,
similar setup machine?

thanks,
mikeo