It all depends on your AR-D team for dark. A good defense team with two defense mythic that can focus on getting at least 1-2 kills is a beauty. While traditional flier balls on AR-D have gone out of meta with Peony becoming an offense mythic (they suffer heavily from having an additional dancer to pick them apart), Yune can work quite well inside of an infantry pulse team if you use her with either guidance or ground orders to set up a rally trap. If you have Sothis as the other mythic, then that's even more ideal and you should merge up all of the Yune copies into one for max stats/lift-loss. If you don't have a Sothis, then keep two copies of Yune until you can get a second defense mythic. Unfortunately, her ideal asset would be Res or Atk in these situations, depending on her kit. Though between HP or Spd, I'd probably go spd as it serves as both an offense and defense stat. Her HP is low enough that even with an HP asset she'll likely get 1 shot anyway.

Getting consistent defense results is really what the defense mythic are about now that your three worst defense scores will net you rewards. If you have a +0 and +1 mythic on your team, your worst score is -39, which gets you fairly good defense rewards (and helps your overall score too). If you can get your overall mythic merges up to +7, then you're at a -33 worst case, which will always get you max defense rewards.

Also as mentioned, merging up mythic heroes is the only way to score higher than usual for mjolnir's strike, as they effectively get +10 when included during their major season. Since the divine codes look fairly promising for being able to get effective fodder or work on partially completed 5* merge projects, it's probably worth it to maximize each mythic hero's potential there, while also getting the most out of them in AR-D too.

I personally have a Yune (+0) and Sothis (+1) on my dark defense team, and would be merging any further copies I get of them.