People reporting Fraxel laser left their skin worse - what's the reason for this?

I'm olive skin male with mild acne scarring. I consulted with few derms and have been told that 3-4 sessions of fraxel will help improve the scaring/skin texture. Research on this site and reddit shows a vocal minority complaining about fraxel leaving their skin worse and in some cases giving them aged look/facial fat loss. On the other hand, doctors here say that fraxel doesn't go deep enough to affect fat underneath to cause aging etc. What's the reason for skin damage then? high settings?