It’s literally a third-party service that let’s others control your desktop. Doesn’t matter how FOSS the clients and end servers are, one also needs to trust the intermediate servers. If those running them are caught dishonest about which country they’re located, the trust evaporates. China or not.
I run syncthing with my own relay and I trust that setup. Owning me through syncthing would basically require backdooring the software, something that’d be likely to go noticed by the syncthing community.
Rustdesk is a backdoor by functionality and it’s already using infra I don’t control. I don’t feel comfortable using that.