What, if anything, is wrong with the junkyard parts shaft?

I had a new one made up from new tubing - and the old yokes - and had it electronically balanced at a SoCal driveshaft/machine shop.

It ran very smooth.

Not too far down the road I pulled the overly high-geared 2.75 diff and installed a 3.70 which necessitated a new driveshaft 1" longer.
(Ford 9" diffs have different companion flange lengths depending on the vehicle they're used in.)

The local speed shop made up a driveshaft from junkyard parts - tubes and yokes, but new U-joints.
It is not balanced and runs as smooth as the original balanced shaft.

I was going to balance it, but like the speed shop owner said, "We make driveshafts for 160 mph doorslammers and have no problems with balance."

From what I see, the problem with some driveshafts is excessive runout, but looking from here it doesn't seem like you need a different driveshaft.

'Course, sometimes a person wants to start a project with all-new parts and there's nothing wrong with that.

Pays yer money and takes yer choice as the saying goes....