That's a good question. Ignoring past headaches you've had with Microsoft's support of open source, how do you evaluate
any open source tool for use in production and/or for long-term viability?
In the NodeJS world, sometimes "long-term" means 9 months it feels like. In open source, sometimes "support" means "file a PR if you care so much about that bug" or "fork it yourself". Are you perhaps expecting a longer term or more support because it is Microsoft behind it?
It looks actively maintained right now, but its published semver is 0.x. There are a lot of 0.x libraries in active use in NodeJS, but it's still semver-obvious grain of salt from the maintainers to keep in mind when considering it for support.
The README tells us that it was directly built to support a production need in Bing today. That seems like as big of a vote of support confidence as you might get from an open source project that a team has vested interest in it.
On the other hand, Bing isn't inside Microsoft's developer division, so they have fewer vested interests in supporting outside developers long term. It's also possible that at some point in the future they get internally sold on a developer division or Windows division or Azure alternative that Microsoft has financial or marketing reasons to commercialize.
If I had a production need for something like Napa.js I don't see any particular red flags to avoid it, it looks like it should be easy enough to migrate to something else down the road if necessary, and would probably consider it.