When you preface something with ‘At their best’ you’re going to say something bad (-ve for the first blank).
When ____ is being pointed out to improve the book —> most likely (constructive) criticism making the second blank negative as well.
At their best, bad reviews are intended to give the author constructive criticism (by pointing out their flaws) so that the author can improve his/her writing. ( I am guessing that the rest of whatever text this is taken from will go on to say that at their worst, they’re intended to make bad writers straight out retire.)