There is one more thing which I would like to understand:

Many people are often saying that you should use High lethality infantry against light/medium armoured foes. I can understand that when fighting cheap units like militias and light infantry who have so little overall defense that the Celtic longswords will work much better in getting a hit and kill. I also know that High lethality units are best when backstabbing other enemy units. But since most spear units have a 14-15 base attack then they should be just as/more effective when fighting against opponents with little armour but a high defense skill (even for a 2-penalty against swords).

Also when I did often test using high lethality units as a frontal charge against all medium/professional enemy units with high overall defense (whether it is armour or high defense skill) then I really did not notice anything noteworthy about the high lethality units in comparison to others, like how Milites illergetum get their asses kicked while the Iberian scutari destroy all other medium units in their path. Maybe the Iberian scutari are an exception to this rule, but I wonder whether you should not just use a multi-purpose spearmen against all enemy units with a high defense skill. Are the high lethality units only useful against cheap units?