Well we needed a coarse, medium and polishing stone and something that had wide applicability to a variety of steels, knives and styles of usage.
So let's start at the 150 Bamboo. It's coarse, fast, and yet still fine enough to leave a good finish. Unlike other series that end at say 400 or 500 grit, I choose this because coarser stones would be too coarse for general use. The finish is pretty fine for a 150, just slightly coarser than say a Beston 500 or Chocera 400. In many instances, you can skip and intermediate stone like a 400 Bamboo, which you can add later if you wish. I could have started with a 400 or 320 grit stone, but the 150 is just so fast that even if you already have another series of stones, the 150 will improve your work using them too. And it's the fastest thing out there for erasing scratches from an Atoma 140 or DMT XXC.
The 1k Ume. The Medium hardness is applicable to a wide variety of blades and blade types. Not so soft as the 1200 for instance. And not so hard as the Ume double hard 1k. The 1000 Black Ume is a larger brick and more costly so a smaller stone seemed a more affordable choice. It is just slightly softer than the 1k Bamboo gold.
The 5k Bamboo - Well it's right in the middle of two 4k stones and two 6k stones, works outrageously nice on carbon steels yet works very nicely on stainless too.
I fully suspect that more advanced sharpeners will add to this starter set as they refine their needs. Basic sharpeners may find that these three stones more than meet their needs.
It is entirely reasonable to add say a 10k Bamboo or 400 Bamboo to this set or even a 60 grit Ume for major repairs and reprofiling. If I were concerned with kasumi finishes, I might choose to add a 2k or 3k Bamboo to this with the 5k Bamboo being a nice followup.
I know this combo won't meet everybody's needs. But that's why there's more than 3 stones in the Nubatama line