Puck.js and Neopixels – Possible?

but I think there's weird capacitance or grounding issues making the signals a bit unreliable.

It's not just you. I just tried it out here (I told @benjaminbenben I'd look at it a while back too) and there's definitely some extra stuff being sent right at the end of the data that'll be confusing some of the LEDs. Hopefully a little more digging around and I'll have a fix for it though.