Regarding your first question: The hive were extremely more dangerous than the vex. When Quria, Blade Transform and her Vex invaded Oryx’s throne world after Crota’s failure and tried to use Vex logic as a way to perceive the Sword logic, Oryx took Quria (who we kill in Season of the Splicer) and threw Crota into the Vex realm telling him to destroy the Vex or return forgotten. Oryx demonstrated his power and was able to repel the Vex invasion, and repaired the hole in his throne world by overlaying reality with his throne world in the Dreadnaught. With the Osmium Dynasty all but fully eradicated minus some children of Savathun and Xivu Arath, the Hive are probably no longer as strong as the Vex. But they once were much stronger. Until now, the Taken were probably the strongest, simply requiring a strong will to control them while being completely loyal, hive-minded, and extremely powerful as well as a diverse crowd of weaponry and abilities There is not enough lore to describe the new race of the Dread to accurately compare, but considering both Rhulk and Nezzy’s backstories, they probably rise to the top and hence we need to wield the Prismatic subclass to defeat them.