I have lots of these in my tank... I started with maybe 6 of them due to an infestation of pond snails, within a month or two. The assassin snails annihilated the other snail population. Their own numbers multiplied since my original 6. They don't require much maintenance and they'll attack any sinking food with high protein content. I also like the way they look, so having a large number of them doesn't bother me much.
In a way yes and no. The assassins eliminate most of the other snails and reproduce according to the food supply. They also naturally reproduce at a slower rate than pest snails and won't reproduce at all at times when there isn't enough food.
I hear about a lot of assassins starving to death rather than taking over a tank.