the armor makes you earn it
Nebula gear does not hand you its damage up front. You generate the pickups by hitting things and then have to go collect them. Each one adds a layer, and the layers fall off one at a time rather than all together, so a fight with steady contact keeps you near the top of the stack while a fight with long gaps keeps dragging you back down.
collection is the hard part
The pickups hang in the air where they appeared and the grab radius is generous but not unlimited. Against a boss that keeps its distance you will lose most of them simply because you never pass back through that spot. Fights where you circle a fixed point suit the armor far better than fights where you run in one direction the whole time.
in multiplayer
Anyone can grab one and everyone standing close by gets the buff, whether or not they own the armor. That makes a single Nebula wearer a group buff. Teams narrow it, though: if you are on one, the share only reaches teammates, so a mixed lobby wastes half the spread.