@Eyal_Shahar Syncing Favorites has been a perennial issue on my Win 10 computers (2 running W10 1909 Stable with Edge Stable, 1 running W10 2004 Stable with Edge Stable, 1 running W10 WIP Fast Ring with Edge Stable, Dev and Canary). I've experienced unwanted duplications, as you have, failures to replicate Favorites across computers, dropped Favorites, and Favorites rearranged for no apparent reason. Edge seems to handle syncing okay for a while, and then acts up. This has been going on since April 2019 and hasn't improved.
In each case I am running Edge "clean" (that is, no extensions or other add-ons or third-party addons except the Font Rendering Enhancer extension to mitigate the "wimpy font" issue somewhat), so I'm reasonably sure that the sync issues are embedded in Edge itself rather than in conflicts with third-party extensions or addons. My Favorites are also relatively straightforward (16 core Favorites on the "Favorites Bar" and 15 news-related Favorites running under "Other Favorites"), with no folders or other complications, so I'm reasonably sure that nothing in the Favorites structure is causing the problem. My W10/WIP computer (the one on which I run W10 Fast Ring and Edge Dev/Canary) was off line for several weeks last month and during that period the problem occurred on the computers where I run W10 2009 (at that time) with Edge Stable, so I'm reasonably sure that the problem is not related either to WIP or Dev/Canary use.
In short, normal troubleshooting does not get to the bottom of the problem. The only possible pattern I see (seemingly confirmed by Josh Bodner's discussion at "Known Issues") is that the issue might be related to the fact that many users encountering the problem have been involved in the MEI program, suggesting that something might have gotten tangled up in the server sync account tied to their MSA early on that keeps triggering the problem.
I run Firefox as my primary browser on all my Windows computers, two Linux computers and two Android devices, and have for years. In contrast to Edge, my Firefox installation uses a complicated Bookmarks nested structure and about 150-200 Bookmarks, syncing across OS platforms. I have never -- not once -- encountered an issue with syncing on any of those devices in the years since I started syncing Firefox (so long ago I can't remember when I started).
I will be testing Edge on Linux if and when Microsoft gets around to releasing a Linux version for testing. My plan has been to move over to Edge as my primary browser after the Linux version has been running stable for a while with the basics intact and operating. I won't do that, though, until the Edge syncing problem is resolved. I don't demand much from a browser (95% of the features in modern browsers are wasted on me), but I do demand that the basics -- security, usability, syncing and so on -- work reliably. When it comes to something as basic as syncing, Edge isn't yet at that point.
Because Firefox is is almost certain to remain my primary browser for another year or two, I don't have a dog in this fight. But my husband currently uses Edge Classic on his two computers, and (according to the trade press) is going to be moved over the Edge Chromium in an upcoming Windows Update. I can only hope that he and others being ported over the Edge Chromium don't encounter the issue. What a mess that would create!
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