Doom + Lina

Why this works

Putting two single-target deleters on the same team often creates a problem of overkill. Doom's entire game plan is finding one hero and removing them from a fight with his ultimate. Lina wants to do the exact same thing, using Light Strike Array and Laguna Blade to burst a lone target. When they're on the same team, you're often investing two huge cooldowns into killing one person, who was probably dead to either hero alone. This leaves your team vulnerable to the rest of the enemy lineup, especially if they have saves or multiple dangerous heroes who are now free to act.

This dynamic makes it hard to secure a real lead. You might get a pickoff with Doom and Lina, but then you have two heroes with their main spells on cooldown, unable to pressure towers or fight again soon. An opponent that groups up can exploit this, knowing you've used your biggest threats on a single target. Your strategy becomes very predictable and easy to counter with defensive items or split-pushing. If that initial pickoff plan fails, both heroes have a tough time contributing to a sustained teamfight, leaving the rest of your lineup exposed.