I assembled some flat pack bookcases for my kids. That was pretty easy.
What was not easy was assembling their bunk bed. The whole frame was steel, and it came completely disassembled. I thought, "this will take maybe 90 minutes." No, it took more like 5 hours, because everything had to be lined up just so or things wouldn't join correctly. That meant lots of loosening and tightening of various screws while I wiggled things around and tried to figure out what was out of alignment.
Whenever I have to move that thing, I'm only partially disassembling it, because fuck ever doing that again.
I draw the line at bunk beds. My older son has a raised bed with a desk, dresser and bookcase underneath, and it was worth every penny of the £75 I paid the nice man to assemble it when he delivered it. My son's bedroom is very small, but that was no problem for the nice man as he knew exactly what he was doing. I would have ended up chucking stuff out of the window in frustration.