- It is outside the JS specification
- Needs to be updated for newer versions of node. Last commit says "Support for Node 0.11.10". Also, it needs to keep up specifically for every platform.
- Will never work in the browser, unlike yield which you can get working on current browsers using transpilers. Increasingly important, as we see more code sharing between client and server (isomorphic JS).
If you ask me, fibers are quite risky.
It works fine with the latest Node for me. Fibers relies on libcoro which has only had minor updates over the past four years.
I'm not sure where you're seeing this code sharing. In practice the only code that is shared is utility methods that don't do any I/O, so whether you use fibers or not makes no difference.
In fact, Meteor, being the framework that is pushing the boundaries of code sharing between client and server, uses fibers on the server.
My co-founder alexlamsl and I are actually working on something akin to Scala actors for JS that is portable between the browser via Web Workers and the server using Node and Common Node (https://github.com/olegp/common-node). We'll blog about it at https://starthq.com/blog/ in the near future.