It's not about difficulty, it's about experience.
After about 5-6 engine swaps plus wiring my current project by hand, I know just about every length of wire that goes into an SR20 harness, I know where they all go, I can identify the sensor just by looking at the plug.
It's not difficult at all, but IMO, you'll be quite confused if you don't know the layout. The SR20 harness is pretty bad IMO, as compared to the VQ35 harness which is literally one strand off the ECU which branches off as it goes around the engine. the VQ35 harness is the most straight forward harness I've ever seen.