I think people who talk about Kingdom Hearts being complicated are the kind of people who want to say the media they like (and understand) is complicated so they feel smart by the implication they understand something complicated.
That said, the themes and mechanics of the story AREN'T complicated, but I think what most people find "complicated" is the amount of characters who are in some way part of another character, a different version of another character, connected existentially to another character, etc, because the variety of methods this shakes out in KH remains to be the most varied way I've seen a single media do. But if you grew up watching YGO DM like I did (or any other media with a permanent "dark" version of another character/plurality themes) then you're already like halfway there in having the fundamental guidepath for most of it. 👍
Also, don't know how much you care for Video Game Videos, but BDJ did a video for Polygon that accurately represents the average person's reaction when trying to understand all of KH, all at once. Which is probably mostly people's problem -- they are probably trying to take it in too quickly, whereas longtime fans had years, spread out, to take in information.
Nomura also did retcon stuff a few times 🙄 and that made things... uh... yeah. 😮💨
Anyway hi I track the KH tag. :S Hope this was useful or whatever. 💜👋