Are you saying that stats work differently than they do in SP? That there is some separate formula added in MP? Andy Kempling's response is vague and he even says he not fully versed in all aspects of the game.
For example "diminishing returns" can mean many things. Naturally, most stats have "diminishing returns." If you have 100 willpower, each successive point is worth relatively less. Or is there something extra on top of this to "soft cap" stats? And how would diminishing returns work with crit chance and cunning?
There are diminishing returns because it is a zero sum game with limited enemy health bars, limited benefit to extra damage, crit chance, and I believe even defense % (needs more testing).
For willpower, the resulting benefit of added .5% attack power is relatively less despite being an absolute increase due to limited enemy health pools and the nature of overkill. So yes, the extra point is worth relatively less, even though it is gradually increasing more.
It is a soft cap because the amount of time required to gain a very small benefit relative to other gear and abilities is extremely prohibitive, and there are limits on how useful the utility of the stats become.
The benefit of increased crit chance decreases geometrically with every increment of crit chance increase. Take for example, the benefit of going from 1% crit chance to 2%. That is doubling the crit chance, or worth a factor of 2x. Going from 2% to 3% is only a factor of 1.5x. This pattern repeats with decreasing benefit of crit chance with every added crit chance. Note, the base crit chance is 5%, and there are multiple passives that guarantee critical hits with status effects Death Blow / Mercy Killing / Knife in the Shadows / Sneak attack / etc. The added benefit of +crit chance is wasted here.
I have not even mentioned yet how the actual utility of crit chance is the crit damage bonus, which once guaranteed is even more rapidly diminishing benefit of extra cunning.
There's more, but I'm out of time for now, and I've mentioned this stuff before in other threads.