Dusk Mane is the only one you should ever use to evolve into Ultra Necrozma, even on special or mixed sets. The synergy that they have with each other is stupid. Steel typing + Prism Armor is godlike and gives Dusk Mane a million opportunities to come in on things that would give Ultra trouble and either wear them down or start setting up with SD, Rock Polish, Calm Mind, Weakness Policy, double dance, whatever.
And the fact that Dusk Mane is also a top tier threat on its own lets you play you mind games with the opponent. There are very few pokemon that can check both forms at the same time so you end up with situations like your Ho-oh or Primal Kyogre outsped and one-shotted by Ultra Necrozma because you sent them in expecting Dusk Mane.
Dawn Wings is just Lunaala but bad. Access to Moongeist Beam isn't enough to justify using it over Dusk Mane, especially not for going Ultra.