Part of it depends on the author, I think, but as a general rule it seems preferred that critiques be done privately. This is especially true for excerpts currently in the funding phase, since negative comments (however constructive) draw attention to issues both real and perceived, and affect how visitors view the project. I’d like to say that anything goes for a project that’s still in the draft stage, since that stage is supposedly for people to perfect their project pitch, but there again, it’s always best to be constructive.