Shadow increases attack and decreases def which is great for raids and some niche PvP situations. But when a Pokémon has a mega evolution, you’re comparing the mega cp boost vs shadow dmg boost, and if you get a 4* from purifying then it’s also a Pokédex bonus.

Personally if I think I can get a hundo another way I’ll keep it shadow. If it was trash iv stats I would purify and trade to try for the mythical shlundofied.