The density of sea water is about 1027 kg/m3 (sea water is more dense than pure water)
The density of concrete can vary a lot but is about 2300 kg/m3(nl)
The density of iron is about 7800 kg/m3 (maybe as low as 7300 if it has a lot of impurities, which is likely for a mushroom).

So when buoyancy and displacement is considered, your 3 ton concrete block's effective weight is reduced by almost half, but an iron anchor's effective weight is only reduced by about 15%. In addition, I'm intimately familiar with the bottom of Oyster Bay harbor which is thick gooey mud (it makes a wash-down pump mandatory equipment for anyone who anchors there frequently). A mushroom will work its way into the bottom -- developing a lot of suction and picking up added mass from the huge mud ball that would cling to it. Also, Oyster Bay is well protected from all sides.

Here in western LIS where all the harbors are mud bottoms, mushrooms are the standard used by most mooring companies.