No more than i worry about my operating system or office suite being closed source.

But then again, i don't put my sensitive information like passwords, ssh/pgp keys, tax returns and stuff like that in Resilio. I very rarely need those documents "on the go". Instead i have working documents, books, notes and more that i need access to, and while i'd rather not share them with the rest of the world, it would probably not make much difference if it was.

Furthermore, i can completely "wall off" Resilio Sync. It runs in a container on my public server, and files i need access to are mounted as NFSv4 shares "outside" the container. Access to the shares is managed through Kerberos.

So even if you make it inside the container, you can (probably) wreak havoc with the files on the NFS shares, but those are backed up, and unless you can find a way out of the container, or a bug in NFS, that's pretty much it.

The container has only the absolute minimum of binaries to allow Resilio to work, so you toolkit is kinda limited, at least when compared to Nextcloud which requies a lot of binaries/libraries to work, along with a PHP interpreter.