Without a good weapon, assassins only deal good amount of damage when they hit a critical strike. So the best  traits are those wich give more critical strike (and critica strike damage) and the ignore armor (shadow strike I think). Equip them items that improve critical strike are a must too, if you find of those.

They have also other nice traits, like dodge, but for more damage I can only remember those as the preference.

Ah I didn't know that mages can crit spells. That sounds rather nasty. Do the crits make up for the lack of getting evoker traits though? My Sov (custom mage) usually ends up with at least +75% spell damage. Though at least this gives me a reason to give her some crit gear.

As for melee I guess what you're saying is that Assassins end up the best for damage at the late game, whenever everyone is geared out and such? And I'll try an assassin archer, though I've been less than impressed with champ ranged damage compared to just trained archers. (I never find good bows, so having a group of 3+ shoot rather than one hero is better for me thus far.)

I agree ...

that´s the main reason why I have given up letting any of my heroes choose the governor path.

Either I let them remain in a city, in this case they are next to useless, due to the fact that they don´t level up and will remain at a low level or the rest of the game,

or I let them go around and kill monsters ... but in this case they level up, but stay outside of cities and don´t support them (and when they´re leveled up high enough, so that they would become a real asset as governor, the game is usually already decided and all that remains are mop up operations, which don´t really require any governors for my cities)