Generally it has the risk of impaction, I have read of some people who have kept large mystery snails with their axolotls and had no problem so maybe that could work out, I'd definitely avoid Malaysian Trumpet Snails, Nerite Snails, or any snails that could potentially be eaten. Mystery snails might, emphasis on the might, be okay, just know that your axolotl could try to eat the snail and die of impaction. I would make sure and have the largest Mystery snails possible because if it can fit in your axolotl's mouth they will try to eat it.