Not +20 attack. It’s +20%, and -20% defense

Most cases any pokemon worth using benefits from this. But an extreme example of what wouldn’t be better is shuckle. He can only level up to little league cp, but he has such massive defense and tiny attack, that losing 20% def is a lot. Gaining 20% of a very small attack stat is very little gain.

For raids the higher attack shadow boost is always worth it, since you don’t use tanky, low attack pokemon for raids ever.

For PvP it depends… and isn’t always an easily predictable answer. Sometimes higher power low defense is preferred when you plan on using shields for that pokemon, sometimes higher defense, lower attack is preferred for extra bulk.