It is unfortunate about the loss of mitigation that never worked properly in the first place, but not Undeath is totally unusable and irrelevant.
Anyone who says Undeath has any use are cheaters who are finding exploits that need to be patched.
3 things I'd prefer:
A) Swap the tier position of unnatural movement and undeath. Then people who want to use undeath will have no recovery and it will be balanced.
Restore undeath to its previous form with A, or people will scream bloody murder.
C) Reduce the stage 4 penalty from 100% to 90%. Still utterly useless in PvP but will let them heal the scratches they received from anything, such as a little fall damage because they rode a horse off many rocks. It's annoying having to dismount and heal repeatedly because of terrain issues.

Seriously, I don't think it should have been nerfed. Just swapped.

As for Nightblade, I have mixed feelings about the change. But it does allow for a longer shadow barrier proc than previously, so I guess I'm ok with it.

I still wish for a few generic things:
A) Magelight, Hunter, and Flare to grant their buffs from either bar, rather than only the active one.
Abilities like Radiant Magelight would show their buff on your bar when you are effected by it. Be it from self or nearby ally who has it equipped. As well as the details in the character screen saying what the buff does, just like it tells what battle spirit does.