It shouldn't be merged into core if it doesn't work on the rest of the platforms core works on. Keep it in some experimental branch until it is ready and more important why would they even submit the PR if a ton of tests are failing?

If it's a compile-time switch to build v8 or Chakra and it's not impeding Node development, there's no harm.

Node is a large code base. The Chakra merge will eventually work and pass all tests - give it some time. One cannot reasonably expect such a large merge to work flawlessly out of the gate. Even the IBM PowerPC port of the V8 engine took many months to stabilize - and it's the same v8 engine.