In my experience they are safe. Most of the ones I have had have been close to full size, so had no need to find a new shell. Others have chosen empty shells.
I never saw them trying to kill my snails for the shells, but I suppose it is always a possibility.
I currently have 6 dwarf zebra hermits in my 45gal, along with nasarius snails, trochus, and ceriths. The zebras have only gone after already empty shells.
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