No, they are not dangerous, otherwise they wouldn’t be prescribed. They can produce side effects because they work by getting the kidneys to excrete excess glucose from the blood, so all that glucose appears in your urine. You need to make sure you are well hydrated to flush the system through. With the urine being sweeter, bladder infections can occur, but this is commoner in women. Watch out for any redness or signs of irritation under the foreskin.
Apart from that, the only significant risk is hypoglycaemia, particularly if you are already on medication with that effect. Metformin doesn’t matter.
All the nightmare effects you can find on the internet are exceptionally rare. Thousands of folk take this without any problem.
Oh, nearly forgot. Don’t drink alcohol, or at least very little. It can worsen The hypoglycaemic effect of the tablets.