I haven’t used it, but my guess is that it is as safe as your use of it. Changing the contents of fields in a MySQL DB isn’t rocket science and it’s probably just issuing SQL statements exactly like PhpMyAdmin does, just in the background.
That said, you could very easily change things in the MODx DB so that your site and/or the Manager become unusable, but then you could do that in PhpMyAdmin too. The real danger of this program is its convenience makes it more likely that people will start casually mucking about in the MODX DB. As long as you’re careful, you should be ok.
It would be nice if it had a global "read-only" setting as the default.
