Like most Pokmon cards, the value of Hisuian Decidueye VSTAR depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (Astral Radiance)
Examples of effects that let you activate library ninjutsu are Primal Growth and Buried Alive
What makes a Pokmon card particularly valuable
In such a case, keeping a slow hand can be problematic, and you should be willing to mulligan slightly more
Often, combo decks run lots of low-cost scry cards to find their combo pieces
The buff Don Andres gives to stolen creatures as well as the Treasure tokens he generates when you cast noncreature spells that don't belong to you are nice, but they're far from enough to make stealing cards worthwhile (especially because the treasures enter tapped)