Lets disregard premade groups.
Its a hassle to to do this with randoms, even when explaining the misson at the start and droping a treasure detector that puts markers on the pickups, I've ended up standing at the exit with 1/4 or 2/4. I get its a "new" misson type but there are sadly the silent players that just run to the exit and dont care to engage with anything else (Its like hitting defection in a sortie and having people defend the life support and thinking they are helping). Also, players lagging behind is more annoying than normal exterminate, since the extraction timer does'nt start till 4/4.
This might be first world problem and will be gone when people learn the mission, but if you want to farm all 284 vainthorns fast, solo is the way to go