Rigidity is usually but not always a good thing. I was a reliability engineer for ten years before retirement and did a lot of Long Term Reliability and HALT testing (Highly Accelerated Life Testing) on a variety of products. It was a lot of fun and we beat the **** out of everything and the engineers always complained that nothing could survive, but that was the point. Find the failure modes in a short amount of time. Anyway, you would be surprised what resonant frequency can do to a supposedly rigid design. Sometimes, adding compliance and altering the natural frequency is a better idea. There are always caveats though, and in a mechanical system like the SBM valvetrain, less flex is generally going to be better at the higher stress levels.
I've always wondered how much a thicker rocker shaft of the right heat treated material would increase stiffness of the system. With pushrod oiling, you could run a solid shaft no? Solid should be more resistant to flex between the hold downs with strong springs. On the other hand, I would have thought it's been tried before and if it isn't popular, maybe the difference is insignificant.