Legos; also the ender 3d printer. I had a nostalgic time putting the ender together. Felt like a lego set. That's not saying it was trivial (it challenged me) but that I trusted it every step of the way not because of previous experience, but because of the obvious well-designed aspects of the experience as I was putting it together. For example: I put on a part sloppily, and trying to attach anther part was the firs…
They, including models with great reputation (e.g. Prusa printers), break all the time. Also the mere fact that there are thousands of mods for all of them on sites like Thingieverse to make them better is another indication they aren't really all that well designed.
Things like manual leveling, heat creep, bottom layer adhesion issues are common problems across all models.
They're finicky things with PLENTY room for improvement.