Detailed damage testing shows that mixed normal+special swords do no less damage than pure special swords. For example, the piercing swords do almost identical DPS to the Brandishes, against their vulnerable targets. It seems that the damage bonus, from having special damage on a vulnerable monster, runs up against some kind of cap.

On the other hand, the damage penalty, from using special damage on a resistant monster, does not run up against a floor. For example, WRH does less damage against gremlins and beasts than DA and the elemental Brandishes do.

So my prediction is: A pure shadow sword would be no better against gremlins, slimes, constructs, and beasts than Acheron/Gran Faust, and would be worse against undead and fiends. That's a weapon I don't need.

Is, that they do split damage because they would be OP otherwise. The only exception are the piercing swords. But with these, the enemies weak to piercing are already pretty nasty, so pure piercing is necessary.
Not sure what three rings was doing with the WRH though....

If you look at when the Argent Peacemaker was changed to pure elemental, thats not true.

The AP used to be split elemental/piercing, the piercing would act as normal against undead. When it was changed to pure elemental, it did more damage to constructs then it did previously to undead, which is what it would do to undead if the undead bonus wasn't added as well. Therefore, pure special damage is more powerful agains enemies weak to said damage type, but less effective against enemies resistant to it, otherwise they wouldn't exist.

I want to believe AP received some extra tweaks, they didnt symply turned the pierce part into elemental.

It's easier to see this with the Hammer vs Divine Avenger.
The hammer has a slighlty longer elemntal bar,but still, it's not long enough to overcome Divine Avenger elemental+normal bars combined.
In the end, they have similar damage numbers on regular swings.