It sure looks excellent, `aft` actually looks very similar to it with the same relay server idea. I don't know the program, but from a first glance,
it looks like it requires a relay server. While I don't think it has any major disadvantages, but needing to communicate with an external service might not be reasonable for some people. And many times you might not require a relay server at all such as transferring on a local network, or from a VPS (maybe croc can identify if its on a local network, didn't dig enough into it). Well, you can host yourself a relay server, but it does require a bit more configuration.
Also, some people might like Rust more than Go ;)