A bit specific, but hard armour joints are always hell for me, particularly elbows and knees. Like, couters and poleyns and shit like you'd see on a full suit of plate armour always takes me ages to get right, even faulds are annoying to draw and all those do is sit on your hips. Something about the kinematics and geometry of overlapping metal plates is just pure agony lol, I am slowly getting better at them though.
Powered exoskeletons are even worse going by my concept sketches, the shoulders and hips on a semi-realistic design are pure nightmares with how they float away from the wearer, but I haven't done a full drawing with that sort of mechanical stuff yet. I suspect mechanical-styled robots would be similarly difficult, that's something I want to practice.