Shitty shits. Literally. As for metabolic acidoses, rates are extremely low (<10 cases per 100,000 patient-years, per https://pubmed.ncbi.nlm.nih.gov/26773926/), so it's almost a non-concern.
I think this depends on the context. For your epidemiologist perspective, or looking at populations, that's a big rate! (Given how much metformin is taken) Would be easy to find people who suffer from it.
From the perspective of "Should I be worried about side effects from taking this drug / should I not take this because it might give me this effect", I think this falls under "be aware of symptoms and stop or weigh the costs if you get it, but you're probably fine".