I wish I can say I have had the same experience ^^
I purchased the plugin thinking it would work great right out of the box, as advertised. 
No where on the plugin page does it say its in beta testing, or even remotely close. 
However, the first paragraph in the plugins Read me is: 

"Note: This plugin is currently in Beta testing. The price reflects this. Once this plugin is out of beta testing, the price will go up. So get the plugin now if you're interested in it before the price goes up!"

Upon adding to my test server and setting up the database ( I used the wrong password mistakenly ) and it wiped all permission data from the test server. -> applied my backup of oxide.groups, verified the correct information on the database and was able to connect. -> At this point all seems to be working correctly. But when the plugin is restarted OR the server, I keep getting this error message stating that the table doesnt exist: 
Can Not Find Any Player Groups In Database! Use the 'ss.uploadplayers' console command to upload all your player groups to the database!
14:40 [Info] [ServerSync] Can Not Find Any Player Permissions In Database! Use the 'ss.uploadplayers' console command to upload all your player permissions to the database!

At the time I am writing I run 8 RUST servers on 4 physical machines and have tried lots of plugins that use databases to synchronize groups and permissions, and all of them have very poor help from the developers and some are even abandoned, I posted a question prior to purchasing and received a reply very quickly indeed.  I made the purchase and must say, am extremely impressed with the plugin. Server Sync does exactly what you need it to do, perfectly.  It was extremely quick and easy to set up and synchronise the groups across all my servers. I opted out of the group perms sync as the same groups have different perms in each server, and some players have individual perms unique to each server. This was so, so easy to set up with true and false being that main thing to configure along with the database username and password.  Total simplicity. I therefore highly this plugin as your groups and perms are totally backed up, in case of a server loss or change you can easily restore groups and perms.   One of the best and most helpful plugins I have purchased, and the customer services are fantastic!  Great job!