Regarding assembly:
First a bit of background: I've assembled plenty of furniture from amazon, Ikea, Wayfair, etc. including a number of pieces that reviewers have described as difficult. While I acknowledge the difficulty of those pieces, I've never found anything too challenging.
Given that background, I would say building these chairs is not easy, but it wasn't quite as hard as some reviews made it out to be. Granted, I went in having read some advice from others here, and there's also a bit of luck involved in that all the pieces I got were made correctly and within a reasonable tolerance.
If you struggle with assembling basic I kea pieces, you WILL definitely have a very difficult time.
That said here's some advice I have for assembly:
* Read the directions once through before attempting
* Keep all screws loose until the end, only screw them in just as much as you have to to catch and be a little secure (like 1.5 turns maybe 2).
* Do all the loose screwing by hand (fingers) or use the provided wrench if you are not dexterous.
* Tighten everything in reverse order, it will be easiest that way. Highly recommend an electric drill for most of the tightening. It's possible otherwise, just probably a mind-numbingly long task to do by hand with the tools. Some of the nuts can only be done by hand/tool.
* The top bar piece I personally found to be the most problematic for fit. I found for both chairs, flipping it around the opposite way I first positioned it worked out better for some reason. That's kind of good practice for assembling metal things with poor manufacturing tolerances: sometimes swapping "identical" parts or flipping them around will make them fit better.
* For the first part where you're screwing the center of the chair to the handlebars, don't bother putting on the second set of nuts until the very end after everything is tightened.
* Check the final tightening with the wrench.
* Dont tighten the upper center piece (with the plastic "J" washer) too hard or you risk breaking it. This is only a risk when using a drill to tighten
I think that's about it! took me maybe an hour and change to do both by myself.