In a moment's whim I took apart one of my friend's burr puzzles. I thought that assembling it would be easy, since it had a clean symmetric shape. That was a big misjudgement.
Each of the 18 pieces is full of asymmetry, and no two pieces look the same. I'm not an expert on these kind of puzzles and even after a good hour of working the best I got was 8 pieces assembled. It was quite clear that I wouldn't ever be able to solve it by hand. And more, it turned out that the puzzle hadn't even come with a solution leaflet.