How hard is it to hide the engine bay wiring, really?

It's not hard at all if you want to hide wires IMHO.

Are you trying to hide wires on a fox or SN?

If it's a fox I did mine over 7 yrs ago and the link you posted took what I did and made his write-up from what I told him (nothing wrong with that) .

Hiding wires, vacuum lines or brake lines is only what you make it, not hard or anything special.

Have a plan... see if it works... then try again.

I hid all my wires and only had to cut and splice 2 sets of wires, 1 set to the A/C compressor and the other set to the overflow bottle and had I not welded up the engine bay I'm not sure if I would have to cut and extended those wires.

its really not to bad at all. Just Put the car on jack stands, remove the front tires and inner fenders and go to work. The passenger side is very simple and consist of only a few plugs. Just take your time and be patient.
Now welding up the engine bay is another story. ........

I did it when I was 17 years old and knew nothing about Mustang wiring or wiring in general!