During my trial I was able to connect 2 subsites to my 1 master site on my dev server and user sync (not with a 33k userbase it wound up failing but it did function). I tried doing this on the live server and it wouldn't work, it would connect 1 subsite and tell me Sync Error and the other site it would say Subsite successfully connected to Master site and Synchronization completed however that was false.
When reviewing the connected sites on the Master Site it would only ever show the 1 subsite with the Sync Error. So I canceled my trial and was offered a one time 30 days for $1, I figured I'd spend this time to try and figure this out.
Since then I have not been able to replicate having 2 subsites on the 1 master even on my dev server where it was previously working. I'm also noticing some odd caching / persistence (even though there is no caching functionality on my dev server). Manually editing the wp_option with a serialized array to support the second subsite doesn't work, the row is properly updated but then immediately returns to its previous value even though unserializing my info properly deconstructs the array so the formatting is good.
Other oddities include uninstalling all options, and then re-activating for a master site, the password won't change, this only seems to change after some time has been passed. I even edited the User Sync plugin files to change the wp_option to wpmu_user_sync_option to force it with a new password.
Ironically, on the master site I have FORCE_SSL_ADMIN but my subsites I did not when it did work originally. After searching through some issues posted here I have set all sites to this option and it hasn't resolved the issue.
And finally, I cannot get any subsite to connect if DEBUG mode is enabled, it just fails repeatedly with the connection error even though the connection will work (for one site) with debug mode disabled.
This plugin I feel could be a huge boon to us optimizing our site network by branching features into subsites but it seems very finicky with many of the posts here unresolved.