I find kits pretty easy personally it takes me usually 2 hours tops to build them, but I've seen people spend 8 hours+ on the same kits.
Only once did I not finish a vehicle the same day I started it(my SCX10), but that wasn't due to me one of the needed parts were malformed in the box, and I had to wait for a replacement from the company to continue.

But as for difficulty most the instructions I've seen are usually well documented, and most have actual size pictures of the screws needed that you can hold screws up to it to get the right size. As long as you don't empty all the bags into a box at once, and only take the parts you need for the step there should be no issue.