As I understand it, certain alloying elements, Chromium in particular, tend to form large carbide crystals, and the alignment and size of these crystals can tend to give the entire matrix a large "grain" size with continuous and distinct grain boundaries. These large and distinct grain boundaries weaken the steel and are the cause of stainless steel's tendency to be brittle at high hardness. Another thing that contributes to stainless lower strength potential is that the more complex molecular transformations that take place during hardening slow the transformations down, allowing, again, large clusters of carbide molecules to form. Stainless, high-alloy steels generally require more complex and carefully controlled heat treatment in order to perform well.
ATS-34 is about 18% alloys while A-2 is around 8%. ATS-34 can actually be quite tough at 61 RcH, but it has to be cryogenically quenched, soaked at a temperature as far below zero as possible, and triple tempered to make the grain structure in the blade as homogenous as possible. Some makers like Jerry Hossom, Ernie Mayer, and Tom Mayo make short swords and machetes out of ATS-34, and they serve well.
[This message has been edited by Steve Harvey (edited 05-25-2000).]