I'm not familiar with rclone and syncthing, but compared to rsync, it can, for example, detect renamed/moved files/dirs and read locked files on Windows (via Volume Shadow Copies). Also, you might have a look at "Which features make FreeFileSync unique?" question in their FAQ:

https://freefilesync.org/faq.php#features

What makes you say that? I've just downloaded the C++ source with the GPL licence.

The only thing that looks to be missing is the "Donation" version implementation of things. Arguably as the author wrote both versions he is well within his rights to set 2 licences.

Of course the counter argument is that he's hiding linked source code (the donation edition implementation), and the GPL states he has to make it available owing to linking, which he's failing to do.

But there's nothing actually stopping me making changes by reimplementing the Donation edition features, and making those available as per GPL terms too.

- Auto updater you probably wouldn't use owing to being a fork and not donating anyway (if you donate, you get the donate version).

- Parallel file copy is easy enough with Boost

- Portable zip is essentially the same as a static exe build

- Email notifications is fairly rudimentary SMTP/email libary use, but would need a basic config dialog.

It's got a UI, so rsync and rclone are out.

Last I tried syncthing I was constantly getting errors on shadowfiles that couldn't be fixed in anyway and it didn't perceptually effect syncing but the constant error condition was not confidence building.

In contrast, I setup freefle sync in windows task scheduler and linked the config file to desktop.

I can now, on the fly, edit the config and keep it tuned to my needs.

Pretty ease to use software.