X factor for one, along with other filler specific concerns.  Once you get a working procedure with a given filler lot/heat/mfg, even a small change can mess the whole thing up.  JS55 may be along here soon, I know he has done a bunch of P91

Not all B9's are created equal. Fred hit it right with the X-Factor. But it is also an issue of resulting mechanicals. It is not an easy proposition for filler metal manufacturers to achieve creep strength, toughness, weldability, and a fully martensitic microstructure (and this is absolutely critical) all in one. Stick with the premier fillers.