Why is that?
My approach has been migrating rather than upgrading. I've set up a fresh Umbraco 10 site, and then started re-creating the content types and date types (for now I've done this part manually), and then have my code re-create all content in the new site and importing all referenced media that I find on my way.
When you create new content via C#, you set set the GUID key that a content item is assigned, so I'm setting the same key as the content had on the old site.
I'm currently working on the third site I'm migrating like this, and it works like a charm. I haven't looked too much at uSync Migrations as I started on the first site before uSync Migrations was out - but I think it has a somewhat similar approach.
So far I haven't handled anything with members or users though.