AFAIR in the end I had 3 boards with issues from Mouser - one of them had a dead color channel on single LED (green channel didn’t work), one or two were completely blank (probably the first LED in the chain dead so others won’t work either).
I had already soldered some loose wires to the SMT pads for testing and my local’ish reseller of Neopixels warns on their site that the LED elements are particularly sensitive to static electricity. So I couldn’t be 100% certain if it was me who zapped them before first use due to careless handling, or if they were broken right from the package.
In any case, ended up ordering new ones from another place at extra cost, and now all Neopixels work fine.
All in all it’s a super easy build (disclaimer: I’m probably close to professional level at shoddy soldering after 15+ years) the case part turned out to be the difficult one, as I decided on an aluminum front plate and a wooden milled case. The milling of wood part is still waiting for some extra free time to appear…