It's very important that if you use this, you call the
pg_start_backup() function before and
pg_stop_backup() function after. Doing filesystem backups is not safe otherwise; even a ZFS or FreeBSD snapshot of the filesystem backed up without those function calls will place the database in recovery mode and may lose transactions.
I would avoid doing filesystem backups instead of regular Postgres backups, both for this reason, and because Postgres backup files (especially in the custom format) are extremely versatile in supporting alternate restores. Since they're single files, they're also less hassle to manage.