But to be fair, that's not really reviewing, is it? Nor is it, I think, very useful to the reader to have only glowing reviews and nothing to differentiate the performances.
There's no problem in being (even highly) critical in a review. My personal yardstick has always been that I needed to be able to make a constructive point in the review, be it negative or positive. The aim is always to help the reader in determining which concerts to visit, to give a little bit of guidance, however subjective.