There's a bug with the promotions that allows a fairly major exploit - it's possible to get 100% withdrawal, and it actually does work that way. In the long term, attaching a great general to a chariot and giving it Flanking I, II, III, and tactics gives it a withdrawal rate of 85%. Once its upgraded to Cavalry, Warchief, or attack helicopter, it has 100%+. Upgrading a Gallic Warrior with Guerrilla I-III and tactics to a Highlander also gives 100%. Upgrading a Caravel to Attack Submarine with Flanking I-III and tactics gives 125%.
While all of these require a Great General to actually work, it is sorta unbalanced. They are literally guaranteed to survive attacking. I don't know how it works, but you should probably find a way to limit the maximum functioning withdrawal rate.
An odd side effect of this is that a unit with a withdrawal rate higher than whatever the maximum is supposed to be can no longer be promoted.