The WS2812B has very tight timing requirements which makes it difficult to drive from the Pi. In effect the data line must be clocked very accurately. APA102 based LEDs are much easier to drive as they have separate clock and data lines.
The only library I am aware of is rpi_ws281x
by Jeremy Garff.
It's interesting that the Unicorn HAT uses SPI. I didn't think that was possible because of the timing gap between SPI bytes inserted by the SPI hardware when configured by the Linux SPI driver. However I think that gap was removed recently (say 6 months ago) which may well have made standard SPI a practical solution on the Pi for driving the WS2812B.
If SPI is indeed now usable for this purpose I'd expect to see new libraries being produced to drive these chips (perhaps they have already, I haven't looked recently).