It's not technically unsafe, but it's unsafe from a UI point of view. It obviously confuses some inexperienced people (witness the confusion in this thread), making them feel helpless and like their work has been destroyed, even if it hasn't. Both git and hg actually have lots of fallbacks so that work is never lost, but without experience, these fallbacks may not be obvious.

Hg's rewriting extensions are sometimes disabled by default for this reason: until people understand how they work, they shouldn't use them. If they try to use them, hg tells them how to enable these features.