I've assembled basses and guitars from parts (necks, bodies, bridges, pickguards, some electronics).
I can do string swaps, intonation, bridge and neck relief adjustments, pickup swaps, and neck and body swaps for most FSO's, so I set up my basses most of the time.
I've found I have weaknesses in assembling complicated electronics (for example, P/J's with tight cavity spacing, Nashville Telecasters with that hellacious "wheel of potential shorts" in the pickup selector switch, etc.) and I'm not good with installing tuners on a brand new neck. (I almost always drill through the front on at least one of them.) I have a tech friend who has bailed me out a few times while discovering these shortcomings.
But as Clint Eastwood said in one of his movies, a man's got to know his limitations. ;-)