I'm pretty much in agreement with a lot of the posts here. I've found that depending on a bunch of factors, my hermits often end up eating the snails.
First, depending on how you acclimated your snails, they could have just died a slow death. Snails can sometimes take months to die from poor acclimation; they need drip acclimation for at least 2 hours. The hermits could be cleaning up the dead, which is a natural part of having a reef system. IMO, in this scenario, I'd rather have the hermits, just in case.
Next, certain snails do better than others. Margarita snails are a colder water snail but often sold for reef systems. Basically in this case, they are slowly cooked to death. These are great algae eaters, but often don't last longer than 4-6 months.
Another thing to consider is that if your hermits don't have enough to eat, they will become opportunistic predators instead of just scavengers. It's either that or the hermits starve and die...
Hermits do need plenty of spare shells because when they molt, they need to find a new, suitable home quickly, for protection. If they can't find an empty shell, they'll find a snail and empty the shell as needed.
One last thing to consider is the type of hermit. Some are more aggressive than others. One of the worst offenders is the blue-legged variety, and this is also one of the most common. There are worse ones out there, but these are the ones that most people end up with in their first clean-up crew, so they end up giving all hermits a bad rap.
Basically, it comes down to being prepared and knowing what hermits you have and how to keep them with snails. I've been able to successfully keep them with a bunch of different types of snails in larger tanks (which is great for when snails actually do die naturally, in case you can't reach them) but found that in smaller tanks they do more damage. Hope that helps.
