Let me expose my ignorance here if I may
Why hardness on a NiCrMo-3 corrosion overlay?
The only thing I can see is perhaps its a poor mans dilution test.
MIC-10 is certainly cheaper than a Niton. But to me that would be way too innacurate, though the thought did occur.
Or, is abrasion an issue in this service as well? More likely. Or high temp mechanicals.
We did have similar problem with overlays on extrusion dies for a pipe manufacturer I worked for.
Though the Ni was chosen for its high temp mechanicals, for obvious reasons in extrusion, and not corrosion.
But if high temp mechanicals is the issue room temp hardness seems of little help, unless you have a reference curve for specific alloy chemistries. Just brainstormin here.

Giovanni,
You make an excellent point.
I think it may be generated from a limited consideration of SCC in which a hardness variance of a specific alloy can be detrimental. But what about erosion-corrosion where the exact opposite would be true? Where hardness would minimize erosion and therefore minimize the combination phenomena.
Isn't this the very reason (or at least a primary reason) behind the Cr in CrMo alloys?