And another thing: are you using CPU of GPU for particles? If GPU, it only handles things that are in the field of view. Setting fixed bounds of a large size doesn’t seem to do anything; in Cascade, it was a way to force particles to be calculated even outside the field of view, but in Niagara it doesn’t, or maybe I’m doing something wrong.
Thinking of it, rain would be too expensive on CPU, depending on the number of particles, so you’re probably are using GPU.