Their main issue is, i think, that they have no flying nor wallhacking mobs in them, making them very cheeseable with the simplest builds.
To be fair, King slime is a paethic boss, even as miniboss. Hes hardly more than a oversi- oh wait, thats exactly what he is.
slime rain isn't a invasion though, plus slime rain can happen pre-hardmode, you need hard mode to obtain the snow globe.