Actually, I'll hazard to say that a review can be absolutely scathing and still be considered constructive criticism rather than flaming. If something doesn't have very many (or any) redeeming qualities, fishing around for some or making them up will not help the writer at all. The difference between constructive criticism and flaming lies in intent: someone who leaves constructive criticism wants to address the objective problems with an author's writing and help that author improve, while a flame is mostly just to lash out at the author for how much they suck.
But these aren't always mutually exclusive. Sometimes an author will be so mind-blowingly wrong in their execution that it's impossible to properly contain yourself. I think the best example of this I've read is the Gen. V fic Darkest Night. It's correct in terms of the basics, such as spelling and grammar, but the way the author handles rape is nothing short of appalling.
(To be honest, that paragraph above is pretty much mostly about people mishandling rape or abuse. Most bad writing is just bad writing, but when you're dealing with something that a reader has every chance of actually having gone through...well, you really have to handle it with care.)
Basically, if you're going to say something an author might find unfavorable...well, you don't have to do a compliment sandwich, but you have to have a study way to back up your arguments.