Yes it is unsafe yet everybody does it. As long as you accept that some backups may be corrupted, it's fine. You'll still have older backups to go to. Of course

Yes it is unsafe yet everybody does it. As long as you accept that some backups may be corrupted, it's fine. You'll still have older backups to go to. Of course, you have to notice the corruption in the first place! On a snapshotting system such as ZFS it should be fine because there won't be a race between updates and copying the WAL file and the DB file. (Haven't investigated it though.)