I had never thought of an epoxy granite structure as replacing a steel fabrication, its not strong enough imo, the yield stress is much lower than steel or cast iron irrc and the modulus of elasticity is about 1/6. saw it more as a way to stop a fabrication ringing like a bell. commercial practice suggests the same, where I have been able to see commercial uses for epoxy concrete, it is either filling the voids in a CI or steel structure OR use as a heavy base for machines deploying negligible forces, ie optical equipment. I would not make a metal working machine just of epoxy concrete, not unless a lot engineering calcs proved my assessment wrong. don't forget to normalize any fabrications going into a machine
Larry you mentioned earlier 50X better than steel and 10x better than CI for epoxy/granite's vibration dampening abilities, seems high to me. I'd read CI is 1x better than steel and Epoxy granite 50% better than CI. Not that easy to find info on the subject though, after all vibration is a magnitude and a decay, its not straightforward, at least to me, what better means, but 10x seems high.
thanks for all the good info being posted on the subject, I continue to believe it holds promise for high end diy machines