The problem is how to get it consistently good over the whole Earth, complicated by the fact that the vegetation density dataset isn't exactly in sync with the color data. That's what HiFlyer asked about - how come the Proland, that apparently uses the same color data, doesn't have this problem? It's because they are using just the color data, and not mixing vegetation and bare land at all. If we used just the colors, it would be the same. But then we would not have the ground mixed by fractal patterns.
The color splitter is driven by the vegetation density value, which is often inconsistent with the colors. One option (apart from getting better veg data) is to extract the "greenness" from the color data instead, or to implement a heuristics that will correct the veg data if they are off for any reason.
I think they need an artist on the team, but for now I'm just gonna drop it.
Artists can't code procedural stuff
Don't despair, it will be taken care of. There are better datasets out there that we will incorporate, and we said the biomes are an initial version.