When double teamed be two humans it's sometimes a somewhat teneble position.  An AI with 50% more ships than you have total makes it very difficult.

In unskilled games, NOT TRUE at all!  I've seen it often!  Even in some skilled games, I've seen the replacement AI do better than the player that quit/dropped.

I just confirmed that a Hard AI is indeed the replacement AI.   But to my surprise, Hards now earn 1.7Xs our income/resources.  I know it was 1.5X in Diplomacy, because I had researched it.  And I know it used to always be the Hard AI replacing dropped players in Diplomacy, not Normal, Unfair or Cruel, as some suggest above. 

To verify, look at the "gains from" graphs (third one down from top, for: Credits, Metal, Crystal).

PS.  Now vs before:
Rebellion... Normal gets 1.1X, Hard 1.7X, Unfair 3X, Cruel 5X, Viscious 7X
Diplomacy.. Normal gets 1.0X, Hard 1.5X, Unfair 2X, Cruel 4X, Viscious 6X
    plus, Cruel & Viscious get research speed bonuses. 

And I believe its a random Hard, so it could equally be an Aggressor, Fortifier, Reseacher, or Economist.  But in my experience, the different types don't make much differences in total play.

Thankyou.  THERE YOU HAVE IT FOLKS! PROOF THAT HARD AI, WHILE STUPID, CAN BE DEADLY TO AN OPPONENT NOT FULLY PREPARED FOR THEM, ESPECIALLY WITH THE ECO SLOT!!!