From my experience it's a vast over simplification to just look...

From my experience it's a vast over simplification to just look at grade. There are many other properties that need to be examined; it's complex:

1. High grade ores are positive but if the interval is narrow the miner cannot get in machinery and as such needs to extract sterile (and/or reduced grade) material around the HG ore - this leads to dilution of the grade - BGL have this problem.
2. As you say the ore can be hard or soft. If is hard (like Missouri) it needs more power to grind it. If it's soft (like Riverina) then it could have clay in it which is "sticky" and fouls up the screens and crushing circuit
3. If the ore geometry is not accurately mapped or the mining machinery extracts waste (or lower grade) around the ore body the head grade will be diluted - i suspect this happened at Riverina and why hey have GPS guided machinery at Missouri.
4. The ore can be "refractory" and thus recovery drops or need special (POX/Albion) processing (like DEG and SBM at Simberi and now Gwalia extensions)
5. The "interval" (or width) and depth of the ore is vital
6. The location (does a lot of overburden or a cut away need to be done) and the amount of waste material that needs to be removed before getting to the ore body

There are many other considerations - as I said it's very complicated and needs a lot of testing and drilling to reduce uncertainty (uncertainty will still exist) but you can get "surprises" on the upside