I have recently attempted the Temple of Lyssa meta with a fairly large group (20-30) and the presence of those players naturally resulted in the event being scaled up to match the amount of players present in the area.
The event then spawned a few of the acolytes, who then spammed about 50 mobs and made it impossible to complete, resulting in many players leaving.
After the players left, the mobs are STILL there and are currently churning out the same amount of mobs, making it impossible to finish for the amount of people present.
Edit: Adds also appear to be the same level as Acolyte spawning them.