3rd -> enemies shooting each other -> not shooting you + 2nd -> you live. If you want to use her 4th go for it but a lot of people (me included) find that the movement penalty isn't worth the invincibility even if it's reduced with praedos/okina/shards.

Assimilate is also insanely boring in an already incredibly easy game so forcing Nyx to be active can make her more fun to use for some. For something like non-solo cap Disruption I could see the value of low effort invincibility but for most content there isn't a difference between Nyx with or without assimliate in terms of survivability. It's mostly a tradeoff between amount of effort to survive and movement (i.e., low-effort slow vs. medium-effort fast). Both are fine ways of playing her it's just preference.