Shadow Shield is far superior to Prism Armor. Prism Armor is a worthless ability on Necrozma, in most situations, as the damage reduction is minimal enough where it doesn't allow it to survive any meaningful matchups. It is still OHKOed by Shadow Rider Calyrex, which it has no tools to survive against. As sumwun mentioned, it has to get hit by 3 super-effective moves to outperform Shadow Shield, though it will faint by the second. Shadow Shield allows Lunala to survive this matchup, and makes a significant contribution to many others. Prism Armor only truly finds use if you choose to Terastalize Necrozma.
Necrozma also misses out on moves Lunala itself learns, such as Moonblast and Wide Guard. Moonblast is helpful in dealing with Dark-types like Incineroar when Meteor Beam has already been used, and OHKOs both Urshifu forms. Wide Guard helps protect Lunala from prominent spread moves, such as Astral Barrage and Glacial Lance. Dawn Wings therefore doesn't have the necessary tools to deal with these Pokémon.
As a result, Dawn Wings Necrozma often finds itself needing heavy support to work in a hard Trick Room team, while its competition Lunala and Shadow Rider Calyrex can be slotted on many other teams with far minimal backing.
