When you consider that the little blighters are hermaphroditic and can lay well over 100 eggs at a time, and then throw in a short life cycle and exponential population growth, you have recipe for escagot soupe.
As for eSHa I've used it a few times and it's worked every time. However, common oversights which can have a negative effect on its efficacy are filtering over chemically active media such as carbon, UV, oxidisers, and resins etc.
And absolutely, I don't include the beloved specimen snails that have been introduced on purpose in my extreme prejudice...just the unwelcome pests that turn up on plants...