>>52339551>Shadow Venusaur and Shadow SalamenceDepends on what Pokémon you're fighting and what breakpoints you're looking to hit. In your average raid with 4+ other people, you're not gonna be losing out because of your lack of one shadowmon over a normal Pokémon.
If you're trying to take on something crazy difficult like duoing Palkia or soloing Mega Ampharos, not only does non-legacy Shadow Dragonite perform similarly as well as legacy Shadow Salamence, but Mega Latios performs just as well in sims as Shadow Dragonite, mainly because of Mega Latios' TDO while still having good DPS. Considering Mega Salamence has better DPS and TDO than Mega Latios, you can more than justify purifying one that's close to perfect (though I'd personally recommend waiting for a lucky trade, raid, research or just general high-spawn event before you purify one).
When it comes to Shadow Venusaur in particular, its typing is actually a problem for it, since it makes it take significantly more damage from ground-type moves, which is mainly a problem because it's also working with less defense because it's a shadowmon, making it generally less useful at soloing Mega Swampert compared to Mega Venusaur, Kartana and even Shadow Tangrowth.
>>52339690>HIGH DPS BUT IMPRACTICAL>>charizard>>aerodactylThese are already fairly impractical as shadow attackers, yet they are the best megas for fire and rock respectively. Shadow Moltres and Shadow Tyranitar are much better investments, especially when lvl. 40 Shadow Charizard significantly lags behind soloing Mega Scizor unless you have two teams of them powered up (very wasteful).
>>scizorBoth are basically useless outside Hoopa Unbound. Even as someone hyper-fixated on stuff like this, I was fine with purifying my 13/15/14 Scyther because Shadow Scizor sucks.
Sorry pic related is so crappy, it already took me 30+ minutes to make this post. 