The issue is that if the VM doesn't support tail calls or alternatives like unstructured goto then you're going to end up with two layers of emulation for such

The issue is that if the VM doesn't support tail calls or alternatives like unstructured goto then you're going to end up with two layers of emulation for such languages, rather than one. That's incredibly slow. Which is all fine and dandy, but people need to realize that WASM will not be nearly as performant as claimed, particularly when hosting other language runtimes.