I have A LOT of assassin snails... apparently one of my tanks is absolutely ideal breeding grounds. LOL
I have 3 assassins in my shrimp tank (keeping the pest snails at bay... they never really eat all of them, just keep the numbers low), they have never eaten my shrimp, not even the one that died.
I have thereabouts 15 in my largest tank... I rarely ever see them!! They keep the pest snails in check beautifully there and eat leftover food (that tank has the most fish so there is usually enough leftovers).
My medium tank has.... I couldn't even guess how many assassins... that I could see... I counted 31 babies... but they are AMAZING hiders so I am guessing there could easily be double that many babies in there..... really not sure how they manage to get enough to eat in there as the tank doesn't get overfed and I rarely even see a pest snail... though lots of empty shells when vacuum.... but something is going really right in there as they aren't breeding in the other two tanks!
I don't think there are any cons unless you want to keep them with other snails in which case it is unlikely the other snails will live...
Once the babies are born, they bury themselves in the gravel for a few weeks whilst they grow bigger. So you could have brazilions of them in there.
I couldn't quite believe it when I first saw my first baby assassin - it was the first time I'd ever seen a baby anything in my aquarium.