Doom + Ursa

Why this works

Thinking Doom's ultimate creates a free kill for Ursa is usually a mistake. Both of these heroes want to do the same thing: get a Blink Dagger, jump a single target, and overwhelm them. The problem is that a Doomed enemy can still be saved or repositioned by their teammates, and more importantly, the rest of the enemy team is free to turn on Ursa. Doom's kit doesn't offer the stuns or reliable lockdown Ursa needs to stick to a target and survive the inevitable counter-attack. They are two solo threats trying to operate in the same space, rather than a pair that covers each other's weaknesses.

This dynamic really falls apart in mid-game teamfights. Doom will find and cast his ultimate on a key hero, but then Ursa is left to fend for himself against the remaining enemies. A coordinated team will simply kite the bear with slows and stuns, bursting him down while Doom can't do much to peel for him. This often forces both cores to compete for farm to get the expensive items they need to function independently, like a BKB or Scythe, creating a greedy lineup that struggles to fight as a unit.