I'm sure a tallow and cocoa butter soap would be firm, possibly even brittle, but does that qualify as "too hard"? I have no idea. What criteria would you use to determine whether a soap is hard enough or too hard or too soft?
Although I've never made such a recipe, I'd be concerned that the resulting soap would be too brittle and prone to breakage once whittled down to a smaller size through normal usage. And being the bubble fanatic that I am, I would also be concerned with the lack of bubbly lather with such a recipe.
There's no "law" against it - meaning a 100% hard oil soap will still be soap. It just won't be very good soap. But if you are curious, then try it! I'd recommend a small batch.
A tallow/cocoa butter soap will have several problems - I don't think it will lather well, because the unsaponifiables in cocoa butter can kill lather and there's no coconut to boost it back up. Also, it thicken VERY quickly.
Although it would be interesting to do some kind of comparison! I would not haves guessed fractioned had a higher SAP value than regular!
I don't have my notes handy, as I am not currently at home, but as I recall, I was not overly impressed with it as a single oil soap, other than I liked it's cocoa fragrance, which did carry through to the unscented soap. Some people don't like the natural scent of cocoa butter, but I really like it myself.How did it lather?