Decidueye also hits, what I like to call, the: "Magic 110"
Meaning, in just two energy attachments, he can hit for 110 damage (90+20 from ability).
This is important for knocking out Shaymins in one hit, and for knocking out most other supporters/basics in 2.
Both Solgaleo and Metagross do not meet this requirement, and also have secondary effects that cripple their ability to attack twice in a row.
They both need support, so having them support each other is just... clunky.
Fates Collide -- 74/124 Common
[C][C] Turmoil Strike
This attack does 50 damage to 1 of your opponent's Pokémon that has any damage counters on it. (Don't apply Weakness and Resistance for Benched Pokémon.)