TomH,
The Heredis file sharing service is not quite like Family TreeMaker or ReUnion. As you have indicated, Family TreeMaker uses some type of web service for backups and syncing. ReUnion uses DropBox.
I have used a few recent versions of Heredis while I was waiting for RootsMagic to release a Mac code compliant version and to be reasonably stable in my Mac environments.
Heredis does not use the web at all to sync two computers. If you read the instructions for sharing very carefully, one will discover that Heredis uses a local network to update files on multiple computers and/or devices. They explicitly state that the computers have to be on the same local network to update files between computers and devices. The computers can be Macs, PCs, Android devices, iPhones or iPads or a combination. As long as each device has a copy of the appropriate version of Heredis, you are good to go.
Up until the last release, the Android, iPhone and iPad apps were free. Now they charge a small fee for the apps. The nice feature about the Heredis apps is that you can edit and update your file on the go, come home, sync your file with your PC or Mac over the local network, and you were done. No manual file transfers anywhere.
For the most part, it worked reasonably well. However, there were a few instances, but very few instances, where I would have to save my current file, shutdown Heredis, reopen Heredis and try the sync again.