I had a quick look at the Powerful ability in the xml, and it says
<AbilityBonus InternalName="PowerfulEmpireAbility">
<AbilityBonusType>Player</AbilityBonusType>
<AbilityBonusOption InternalName="PowerfulEmpire">
<DisplayName>Powerful</DisplayName>
<Description>All units get +3 Strength and +2 Constitution</Description>
<Icon>Ability_Powerful_Icon.png</Icon>
<GameModifier>
<ModType>Unit</ModType>
<Attribute>AdjustUnitStat</Attribute>
<StrVal>A_UnitStat_Constitution</StrVal>
<Value>2</Value>
</GameModifier>
<GameModifier>
<ModType>Unit</ModType>
<Attribute>AdjustUnitStat</Attribute>
<StrVal>A_UnitStat_Strength</StrVal>
<Value>3</Value>
</GameModifier>
<Cost>1</Cost>
<HideWhenUpgraded>0</HideWhenUpgraded>
<AIData AIPersonality="AI_General">
<AIPriority>5</AIPriority>
</AIData>
</AbilityBonusOption>
</AbilityBonus>
I believe it should not say A_UnitStat_Constitution, but just UnitStat_Constitution and the same for strength.
I didn't check these, but the same XML problem exists in some other faction traits for units, such as Quick, Weak and so on.