Haha, that's the issue with several crates! You see it's voluntary work and hence, being part of the open source community, it's our responsibility to submit issues and create PRs! Or maybe write a lib your self, and open source it so that other people in the community can use it too.
Do you specifically need SSE? Websockets offer a similar (arguably more general / flexible) bidirectional communication channel. If you can broaden your search to include those, you'll find a number of other platforms become viable.
In particular, Actix-web, which might suit the game model well.
The downside with websockets is that they (AFAIK) don't integrate very well with proxy and firewall. I read about people that have to make fancy things in order to make websockets work everywhere and since I am not such a network expert I would like to avoid to fall in those traps