Skywrath Mage + Viper

Why this works

Combining the slows from Viper and Skywrath Mage feels like it should create an inescapable kill zone. Both heroes pour out a high volume of magic damage, and in the first few minutes, this can certainly bully an opponent out of the lane. The problem is one of redundancy and a lack of true lockdown. They both slow, but neither has a stun to prevent a Force Staff or a teleport. This means their kill threat relies entirely on overwhelming magic damage, which becomes much less effective once enemies start building basic magic resistance. A single Glimmer Cape can often foil their entire gank attempt.

As the game progresses, this weakness becomes more pronounced. While their laning presence is strong, they struggle to secure kills on cores in the mid-game. A farmed hero with a Black King Bar can often walk right through their spells and kill them both, since neither Viper nor Skywrath Mage is very mobile or durable. They end up being good at killing isolated supports but can't provide the control or diverse damage types needed to handle enemy carries. They're forced to play from ahead, because if the game goes even, their specific damage profile gets countered too easily.