Another definite vote for POANG here, too. Back in the day, I was around 120kg, and sat in these regularly without mishap. I am sure that if you absolutely hurl yourself into one, it might be possible to break it, but IKEA have always been very hot on furniture testing, and I'm sure they're up to at least their rated loads.
I do get that, for a lot of people, not having 4 legs firmly on the ground is going to feel pretty insecure, though - but for me, they're one of the comfiest chairs I've ever sat in.
It looks like there may be a vulnerability where the arm connects to the seat at the back, and I can definitely confirm that the screws that attach the two do have a tendency to work loose over time, so an annual tighten-up with a suitable hex head is worth doing - I do wonder whether these breakages are because the arm and frame are no longer anchored together in shear. I keep meaning to find my Loctite and pop a bit onto the screws, but it's so much not an issue, I never get around to it.