Nock works perfectly, of course, but it's poorly optimized. The various "jets" (optimized implementations) in our runtime haven't been adequately tested and there are probably a few corner cases, errors, etc, where the behavior doesn't match. Even the new Nock runtime (I just rewrote more or less the whole interpreter) is probably a little too bound to Urbit specifically.
But the system has been self-hosting for quite some time, and there's certainly no doubt that the general approach works...