It is not the reviewer's job to offer any kind of negative criticism.
That would only show a lack of maturity on the reviewer's part.
Same goes for play critics, restaurant critics etc. etc..
Those who are good will receive positive reviews.
Those who are not, do not need the added burden of negative reviews to further impede their progress.
I happen to be one who gets a lot of enjoyment out of watching a performer hone his/her skills over the years.
The seasoned vets, I enjoy in the privacy of my vehicle or at home with a glass of Irish and a cozy fire.
But we all started out somewhere.
We all sucked at first.
We are the product of a patient audience and should have it in us to afford the same patience to the next generation.
Thus, the critic who takes it upon him/herself to insult those who do not meet their self imposed standards are completely missing the point that no artist has ever peaked.
The act is a living, flowing form that in most cases will become a masterpiece to some-one.