I am no expert in ammo or medical stuff. The value I see in milk jug shooting is the fact that it is a constant medium and you can compare different bullet types and loads, assuming the many other variables are kept constant, like distance and barrel length.
I stumbled on this web page here
terminal ballistics
that has a LOT of bullet expansion comparison results from shooting a lot of water-filled milk jugs. (No association or affiliation with the author.)
I guess if I was looking for a bullet that expanded 'somewhat' and someone showed me results of 3 different bullets shot thru the same milk jugs, and one didn't expand, one expanded a little, and one expanded a lot, I might consider the one in the middle. At least there is something to look at and compare.
Now, how this compares to shooting a body, I don't know. A soft tissue shot is going to be different from a bone-type shot, and then there is placement, and distance, and more variables.
I take milk jug shooting as interesting, of some value in a comparative way, but not a guarantee of anything except what happens when you shoot milk jugs full of water. Shot placement in defensive shooting may very well be more important than bullet type, hopefully I'll never have to prove or disprove that.