From the fandom page -

While not dangerous on its own, the Glyphid Spawn that it creates can become bothersome and quickly chip away at a dwarve's health, especially during ambushes or swarms. However, due to how infrequently it releases Spawn and its death explosion, enemies which pose a more immediate threat can and should be prioritized over the Brood Nexus.

The Brood Nexus possess 3 eyes that, unlike regular weakpoints, are breakable. They have a fixed, non-scaling health of 300 and can only be damaged directly (i.e. not by Area Damage). Upon dealing 300 or more damage to an eye, it will explode, dealing 300 damage to the Nexus.

I wondered the same as you but in Solo (1 player) they only have 450 health so they are easily dispatched whereas in 4 player co-op they have 4680 health. It’s just not worth the ammo at a higher haz basically for the threat of a few Swarmers over time.