Not all reviews are created equal. A review that is based on a careful reading of the whole manuscript, and gives specific, actionable recommendations about how the paper should be changed, with clear justification, is far better than a review like a movie review that simply throws derogatory language at the paper (some highlights from reviews of my own papers: "shallow", "weak", "very, very bad") or rubber-stamps the paper despite serious problems. I think it's often worthwhile to spend time checking calculations and checking that cited papers really say what the authors claim they say, although such things are beyond the call of duty in most cases.
But, there's little incentive for writing good reviews or punishment for writing bad ones. All reviewers get out of reviewing is getting to list in their CV what journals they've reviewed for. So, inevitably, careerists write a lot of sloppy reviews. Also, as mentioned in a comment, unscrupulous professors may take the credit for a review written by a graduate student.
What I would recommend is, don't worry about how many reviews you're completing. Spend the time you think is necessary to write good reviews, and if that only gives you time to write a few, then good on you for putting in all the effort to help those papers achieve their potential.
For what it's worth, I'm a postdoc in public health and psychology and I review maybe 1 paper a month tops. Each takes me at least a day of dedicated work.