My take: It's nature at work.
I like hermits, so I'll probably always have a few in any given tank. I don't really ever remember them having a direct or significant impact on snail populations, but then again I don't exactly count snails on a regular basis. If I notice any type of critter getting low, it's not a big deal to spend a few bucks and replenish the stock.
That said, my cleanup crews are typically vastly smaller than the "advertised" sizes you see suggested at online vendors or in the forums. I've seen people suggest one critter (snail and/or hermit) per gallon. By contrast, in my old 125g, I probably had two dozen mixed snails and 4 or 5 hermits. The 55 I had before the 125 had maybe two or three hermits and maybe 5 snails. The 29g and two 30's I had before the 55 didn't have ANY cleanup crew in them.