We had another thread about it and I think its kind of a mix
heralds give a much higher chance to drop sunders than any other mob per kill, but they're sporadic enough and can take so long that they're already only a moderate improvement over andy/travincal at farming sunders, and heralds drop absolute shit other than sunders. So its many orders of magnitude more rewarding to farm andy or travincal than to farm heralds, and heralds are often so high life/dps that its not even worth killing them for experience when just grinding tz for levels. At tier 4+ especially, the best move is to just skip them