Beloberto
Bug Catcher
I see. Thank you!It isn't the greatest type in a vacuum because they don't defensively synergize well, outside of losing Grass's bug weakness thanks to Ghost. But it is better defensively than Grass/Flying because it lacks a x4 weakness and a Stealth Rocks weakness. Offensively Ghost/Grass is a good type combo, very few Pokemon resist the Combo. This is less because it is a good type combo and more because Ghost is just a good offensive type in general, only being resisted by Dark and with only Normal types immune to it.
This means that Decidueye's STABs are only resisted by a select few Pokemon:
Grass/Normal
Grass/Dark
Bug/Normal
Bug/Dark
Fire/Normal
Fire/Dark
Poison/Normal
Poison/Dark
Dragon/Normal
Dragon/Dark
Steel/Normal
Steel/Dark