I don't think criticism is helpful unless it is asked for (for instance when a quilt is submitted for judging), so I wouldn't point out problems in workmanship. But a block turned the wrong way, a fabric that is upside down... any problem that you legitimately think is a mistake that the quilter has not seen but would like to fix... I don't think of that as criticism. I would privately and politely ask if the quilter knows about it. I know that I have made those mistakes before, and I would like to be told so I could fix them before quilting.