I have a bunch in my pond but they were more decorative and just another form of life than for algae clean up. I have approximately 2000 gallons and based on the average that I read of 1 snail per 20 gallons of water, I didn’t really want to add 100 snails. I also have lots of nerite snails, they seem to be go after algae more than the plants, and they wont reproduce in the fresh water.
I think it is like adding anything new to the pond. There is always a posibility of getting something from the breeder or store that you dont want in your water.
As for the balance of your pond, I believe it is similar to adding misquito fish. They have benifits without much in the way of negatives and they shouldnt contribute much of anything to the load on your filter.