I realised this while playing today when both myself and my friend died at the same time in a game of dominion. He was playing an assassin and I was a heavy. As soon as we spawned we went running for point C (the furthest away one) where our two team-mates were holding off 2 of the enemy (with the other two dead). He reached point C just as I was approaching point B. The two enemies that were dead had re-spawned and had joined the fight making it a 4v3 until I made it there, by which time my team had lost more health and one guy was down, meaning it was a tougher fight for me when I got there.

The reason this is unbalanced is because all characters are supposed to be balanced when fighting one on one. However fast characters can die and get back into the fight faster than heavy characters. This means that even if the heavy characters are balanced with the assassin characters when fighting, they're still at a disadvantage over-all tactically.

This obviously doesn't apply to the duel mode but it does apply to all 4v4 modes. Quite simply heavy characters should have an advantage to counterbalance the disadvantage of their slow movement speed.