Hmmmm. That's an interesting point.
Law 41.2.1 "The umpires shall be the sole judges of fair and unfair play. If an umpire considers that any action by a player, not covered in the Laws, is unfair, he/she shall call and signal Dead ball, if appropriate, as soon as it becomes clear that the call will not disadvantage the non-offending side, and report the matter to the other umpire."
Nothing there that limits 'unsafe' to reasons of light or weather. To be fair, though, there is nothing in the laws that refers to safety - only fairness.
Laws 41.6 & 41.7 then go into far more detail on the subjects of "Bowling of dangerous and unfair short pitched deliveries" and "Bowling of dangerous and unfair non-pitching deliveries" respectively, but essentially it is up to the bowler's end umpire to make such [subjective] decisions.
Quotes taken from the MCC's website,
here. Laws
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