I really thought that a paid membership to a plugin developer group would result in better results, but to no avail.
On the first site, it just failed and about 30% of the users were transferred.
Now when I do it, I get "Synchronization of all Subsites completed." and not one user gets transferred. Tried it on both ends.
The time it will take me to work with you to "debug" your own plugin is probably equal to me coding my own solution.