I agree, I do have issues with sync. In an ideal world, I could just mindlessly use Joplin on any platform I wish, and automatic sync would just take care of all the coordination for me. Alas, in my experience, such is not the case.
In addition to the examples that you provided of applications where sync just works without constant hand-holding, email is another. I use Gmail and IMAP, so all my email is kept on the server and synced to all clients. I've accumulated this email over 30 years, so I have a sizeable collection, though I try to be diligent about cleaning out unnecessary cruft. Google tells me I'm using 2.63 GB of my allotted 15, and that is almost all email. I have never worried about syncing my email from multiple client systems. Gmail / IMAP just takes care of everything. So, bombproof syncing does seem to be possible. Hmm, maybe that is something to explore: using IMAP for note taking and syncing. Email is fundamentally different from notes, though. Email is seldom amended, and each email stands on on its own, though of course we do have email threads. Something to think about, perhaps. Or maybe time is better spent figuring out how to make Joplin's syncing more robust.
And I also agree my notes database is small. I don't feel a need to store large binary attachments in my notes, so they are almost all just text. And I've established a habit of not just dumping all my "knowledge" into Joplin; I keep the bulk of it on my primary home computer, and just make sure to take regular backups to my NAS. So, the only items I intentionally put into Joplin are those that I anticipate I will need on various platforms - principally my main computer and my cell phone.