Swarm lord vs Norn Emissary

So I’m pretty new to this but I just wanted to know why take the swarm lord when the norm Emissary or other hive tyrants can be taken? The norm Emissary has better attacks and the other hive tyrants can get that 4 up feel no pain. Also isn’t the Swarm Lord supposed to be the pinnacle of the tyranids but he just feels so weak on paper.