RE: Worst part of assembling an ARF is....
I think the worst has to be (as mentioned several time) correcting mistakes of the manufacturer and of all those the worst is:
1. WARPED WING
2. WRONG INCIDENCE
3. WING AND STAB DON'T LINE UP
Control Horns are a pain as well but with the right tools, it's also acceptable. Except when the screws are twice as long as is necessary!