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.

The Dread. They're of stronger paracausality than Taken or Hive because they're directly created and empowered by the Witness.

Now that the vex can simulate the paracausal, thanks to Asher creating the veil in the vex network, they would easily be the most dangerous foe. Not to mention that they can convert guardians, something not even Oryx could do with his taken powers. Even Calus has mentioned that we have yet to face true vex warriors

It's not really dangerous. Dying to be reborn in the light, imagine a faith doing that...a faith with tennets like: Inspire courage Courage (long story short) leads to death But hey you'll live again Just your sense of self, not memories, just the fundamental you My faith is pretty destructive too. Edit: Oh yeah, we may have learned to lie along the way. So, how much y'all know about us, may not be the entire truth.

Elsie said that in the context of that particular time, the Vex were about to become paracausal and if we didn’t stop them they would become stronger than the Hive. That’s the issue with assessing threat. The Hive are presently a greater threat. The Vex only become a greater threat if they gain paracausal power Which is likely going to be the centerpiece of their little episode or whatever.