Mystery snails and the like will breed quickly. If you set up a planted ten gallon with cherry shrimp and snails...my tank was very self sufficient. Since it was heavily planted and only had shrimp and snails, I rarely did water changes and had a simple sponge filter. It took a few months to get established but once it did, there were dozens of snails that became an annoyance and hundreds of shrimp(from an original 30).
WC are often carrying internal parasites, such as trematodes and the like, that do not affect the snails at all. The parasites stay dormant and wait to get eaten to mature in the intestines of fish. I just wouldnt trust wc