(NOTE: The commentary below specifically discusses the +3/+3 trait of Tarth, NOT the Tarth race. I only realized I should point that out after I had finished writing below.)
Not to be devil's advocate, but I steamroll every game as Tarth, BECAUSE of its racial +3/+3. Using a custom Tarth of Quick + Lucky + Master Archers, and creating Archer units using just the Tier 2 bows and giving them Quick + Finesse (a mere 16 additional production,) you're looking at groups of archers with 23 initiative and 18 additional attack at the smallest squad size. Squads that can fire a double volley off before anything but high level monsters have a turn (which in this case, is 96 attack.) This is BEFORE you account for wargs , initiative increasing magical equipment, or for creating them from a location with the +initiative per essence spell (Aura of Grace, IIRC.) The stats only get more bonkers if you start accounting for Charge or Bloodlust, or even swapping Master Archers for alternative super weapons like the Athican Longsword or Dual Great Axes.
Now, I'm not saying the trait is perfect, but with the exception of ONE game (I play on a minimum of Challenging, depending on how much b.s. I want to deal with when land-grabbing against the AI,) where Ironeers sent seven full armies of 9 units against me, I can't remember a game where the above scenario didn't play out entirely in my favor. In fact, if an enemy army has 6 or less units in it, I can generally expect to take no damage, killing off the large movement units off first, ranged second before they get an attack off, and melee stragglers last - well before they make it into range. At 7+ units, I have to prioritize, but the fact that it usually takes 8-9 units to even damage me should state something about the power of this trait.