My assassins have on rare occasions taken out a mystery snail. It all depends how hungry they get. My assassins love veggie sticks but if I forgot to drop one in every 2-3 days they start to look elsewhere for food. They do a great job on mts. I've never found a bigger mystery snail shell empty though, it's usually the smaller ones.
Who knows though it may be part of the assassin gang initiation to take out a big snail. Lol