Dunno about red flags,but if something is poorly edited it won't allow the reader 2 read cohesively,clearly. Reader will get stuck on bits that don't make sense,that are superfluous,that clash,that don't flow smoothly. Storylines that don't work. Awful undeveloped characters, poor grammar,spelling.

Sure but at some point there’s only so much you can do as an editor, like if you get a grossly underdeveloped manuscript, something that screams page-oner, do you consult with the writer to consider a rewrite or do you do what you can to improve what’s there??