My main complaint about GIMP is I have attempted to use it about 10 times in the last 20 years and I have never actually understood how to use it. I realise this arguably says more about me than GIMP but I also think I'm in the upper quartile of computer competence and I did an undergraduate module in computer graphics - surely this is it's #1 issue. But this is where I understand the paradigm of how its UX operates is beyond reproach.