Whether snails cause any trouble or not varies from fishkeeper to fishkeeper and even from tank to tank. I've not found it difficult to keep on top of mine, but some fishkeepers face a continual battle.
Basically, they are harmless little creature in themselves- they certainly can't hurt your mollies- but if there's too many of them, they become a nuisance. Like all animals they produce waste, and they can eat your plants. The best way to keep the population under control is not to overfeed, as they will reproduce in proportion to how much food they find. And you may want to scoop out the ones you find from time to time. I find I always catch a few unintentionally when I do my gravel vacs.