Is it impossible to get valuable critique on such a forum?

Hans, I think that is excellent advice.

A case in point; a local amateur photographer has been posting her work on an art selling site (I won't name the web-site, but it is quite big) for some time. Inevitably, every shot (often a landscape) has attracted the usual cheer-squad of supporters with, as you mention, "awesome", "fantastic capture" ("capture" is a favourite term as it suggests  great preparation and relieves the commenter from posting something more analytical), "stunning" (a favourite), "wonderful" etc.

Buoyed by such incessant praise for  mediocre works (negative or critical suggestions are verbotem), she arranged an exhibition of her masterpieces in a gallery at the capital city of the state. Unsurprisingly, none sold. Not one.

Now she's almost disappeared, no more posts, reality has finally arrived, and the cheer squad has most likely found another "victim".

I've noticed over at Steve Huff's site something similar happening. Snappers are encouraged to send in a small portfolio. After what seems a required lauding of the site and Huff, up come the snaps. Mostly they are the greatest photos ever published, according to the cheer squad, well, until the next lot arrive.. A more sober assessment would mark them as very ordinary, with occasional exceptions.

Commenters were recently "advised" to be positive and indeed they are, tumbling over each other to display their enthusiastic positivity as you would expect if it is presented as an absolute virtue. And woe betide anyone who offers criticism. Unfortunately, it has done very little for anyone learning how to progress.