Yes they are, watch the trailers or the episodes, kids who haven't even graduated combat school walk all over them. The only time grim are ever portrayed as an actual threat is in the world of remnant shorts and when velvet talks about how "there were so many of them" all they have is numbers and even that doesn't seem to be problem when they break through the tunnel at the end of volumn 2. The only character to ever die from grim is torchwick. You know, the weakest villain in the whole series, and he had his guard down. I bet even he could have survived if he wasn't fighting ruby. You could argue the only reason he died was because ruby is the main character and there was no other way for her to win without pulling that comedic finish. The idea of grim being scary is laughable once you realize how little damage they actually do.



Sorry for the wall of text



tl;dr Grim are weak, the show proves it