Most of the time is spent fighting with Stud.io (the software) and dealing with its bugs (it runs OK in Bottles (i.e., managed Wine environments), but stupid rendering bugs mean you may have to do the final export step on a device with a different GPU).
When you also want to print a booklet of instructions, getting the generated steps neatly out of Stud.io's instruction maker and into a sane pipeline for making the print-ready PDFs takes some work (as well as CMYK conversions), but that is mostly a matter of setting up some scripts to wire up pdftk and ImageMagick and friends.
My son and I were assembling a set, and one of the corner pieces was strangely absent. My son asked, “why can’t we put this piece in yet?”
The answer came when we turned the page and saw the model had to be flipped. That missing corner piece was the only thing we could use to guide our placement of pieces on the underside of the model.
Similar things in more complex sets include using different, bright colours for bricks on the left/right sides of the internal parts of models. It makes it easy to keep your place as the model rotates through the steps.
However, I agree with the other poster that you can get good at it. And you really have to do it anyway, since creating instructions will find bugs in anything but the simplest models and you want to solve those before paying for the pieces.