Synching data is hard
Synching data is hard but MS have at least six products that do it.
They have always been a bit crap at it and I don't know why. For example in the early noughties AD struggled to work at all whilst eDir finessed bi-directional sync with multiple partitions. One Drive is pretty old and yet it breaks as often as an Outlook profile (which is also old and relies on local sync). Nextcloud (and Owncloud) camly carry on.
I'm fairly sure it is a combination of over engineering and minimal testing. DFS-R anyone?