>>51769502The way that shadow bonuses function is like a permanent +1 attack stage / -1 defense stage, it doesn't change the underlying stats.
So naturally, Pokémon that already have considerably high bulk that leans more towards HP than defense benefit more.
In some cases, a shadow Pokémon can outperform their mega evolution.
This is the case for Shadow Venusaur, in gym and raid battles it's considerably higher DPS than Mega Venusaur. TDO suffers, but you can have multiple Shadow Venusaur, and only one active Mega Venusaur.
Also, Shadow Pokémon can't Mega Evolve, they require purification.