I think that your desire to critique is more than fine. I notice that folks have their own opinions of their work. If they like it and you don't, then typically they will deem you unable to get what they are expressing. Contrarily, if they really don't think their work is up to par, no matter how much you love it, they will only figure it "will do" at best. The biggest worry I would guess would be to criticize someone who doesn't think they are up to par, and may be over sensitive to boot. But these people tend to wash out one way or another.

Besides your opinion as a viewer is just as valid as the artists', maybe more. I gather you are also a photographer so you are worried about seeming superior; but would you say that an average viewer shouldn't voice an opinion? Then you should also be able to do the same. I think, short of being rude, the photographers' responses to your critiques are too personal for you to feel responsible about. If you don't respond, that could be just as hurtful, as could insincere support.

Be yourself, say what you think; that is my vote.