SE are even more than "what you see is what you kill" for up to medium sized Grimm, because you get a spherical kill zone with the SE user in the center. There is no overwhelming with numbers, because if you ask how many are killed the answer is all of them. Starting from zero Ruby even has at least one reload.
A limitation until now is that Ruby is so selfless, she cannot fire in self-defense even if faced with a Godzilla at zero range, but Maria could do it.
Counters to be employed by the opposition would be taking cover or wearing protective (speak opaque) clothing and not looking when Ruby fires which is basically the same.
The sky is coverless terrain and Salem's flying apes have no armor, so they only have a chance if Ruby "jams" or is herself placed in an opaque container.
I would like to see Ruby + Jaune in a lighthouse to focus the light of the SE, that should be sufficient to whack any Grimm. Fortunately for balance a lighthouse mirror system is not easy to transport.
By the way, what happens if Ruby pinned a superlarge Grimm and then Weiss finished the Grimm. E.g. hack off the head with her super-sword strike. Would it be added to Weiss' summoning pool?