This does look interesting. I don't see a difference between this and any other server-side templating language though. I actually need an isomorphic representation on the client and server. (this is what makes renderToString() valuable) I guess I could be extremely careful in writing the server output, and writing the client output to match, but at that point, I'd be writing it twice... and I could use any language... unfortunately, it'd probably end up being Handlebars.
It's for client side, not for server side. It's compiled into Wasm and delivered to client. You can use same code for server-side pre-rendering (for faster loading or for search engine indexing), but it is not main goal of Yew.