Slark has always been a very strong carry if picked in a good game. He has a ton of regen after he has his ultimate, rendering him relatively free from going back when flash farming the lanes or the jungle. He also doesn’t have much mana problems as his first skill, Dark Pact, doesn’t require tons of mana to spam.
Few patches ago, Slark was almost unplayable. With his new nerfs disabling dark pact from breaking Hex, Slark struggles to even pick off Shadow Shaman in high level games. With the recent patches, he has gained many buffs such as item choices, improved turn rates (one of the highest right now in DotA), and also stat buffs. Making him once again a very viable carry choice in a game that doesn’t have much AOE spells.
Overall, I would say pick Slark when you see opponents picking heroes that do not have alot of escape mechanisms such as Zeus, Crystal maiden, Orge etc. These heroes will typically be feeding you if they are ever alone. Heroes that are generally good against Slark are those with high bursts such as Shadow Fiend, Sven etc.