There's always exceptions in popular medicine. No doubt a small minority of people get no benefit, or may even be mildly harmed by the medicine. But Metformin, in particular, is one of the most popular drugs in the world, and it's strong safety profile is well established. "All-cause mortality is lower in people on Metformin". So, I don't think there will be any big surprises found in its safety, but maybe we'll find that a certain minority of the population aren't suited for it.