Lots of people may well benefit from it, but given its rather serious side effects (well beyond diarrhea), it would be quite extraordinary if there was nobody taking it who shouldn't be.
For example, metformin interferes with androgen receptors. This makes it more difficult to maintain, let alone build, muscle mass. But building muscle is one of the best ways to increase insulin sensitivity, and it has a bunch of other health benefits as well (increased bone density, better balance, protein reserves against health problems, etc). The loss of muscle mass can compound a lot of health problems, including those from diabetes.