If I see a car streaming smoke, I think it's more urgent to tell them that it's streaming smoke than it is to do some research to figure out what could cause it and how to fix it before telling them. If my friend shoots a short film with the punchline being a guy in blackface, I probably want to tell him, "Please don't use blackface," before he takes it to the film festival, even if I can't come up with a replacement joke in time. He's the goddamn filmmaker, he can come up with his own replacement joke.
As far as adding compliments to your critiques? Sure, there's benefits. We're not robots. A person is more likely to want to make a change and move forward instead of curling into a ball and feeling bad if you give them some hope that their work is worth continuing and improving on. But I don't think we should be forced to match every negative criticism with a positive statement. After a while, it becomes obviously forced ("Well, I like your handwriting here"), which is even more demoralizing than anything negative.
"So your fly is open ... but I like your, uh, hat?"
Maybe a good rule of thumb is to make sure at least one genuine positive comment goes along with every "package" of feedback you deliver (like one meeting, one email, one post-coital conversation). And deliver your negative feedback with a tone of, if not reluctance, at least not glee. But you don't need to pay a compliment with every damn typo you spot.