Harmony - Troop Symmetry

Luck without Spellpower doesn't have enough predictability, which isn't taken into account in the average damage value, but it reduces its value.

It's effective in dealing damage, but not in the final stage. Because unpredictability works both ways, it's harder to defend against luck.
Also, the value of luck is significantly increased if the opponent is stronger, and decreased in the opposite case.

But overall, the bonus from the Elven Spellpower is +40-60%, and even less, considering the racial bonus.
In a battle against only sworn enemies and with 1 city, the damage increase from Spellpower is +175% (125% + 50%), without Spellpower it's +87.5%, so the difference is 2.75/1.875 = 1.47. With artifacts that give +2 luck (up to +5), it's already 1.29.

In short, the effectiveness of Elven Spellpower can be considered to be around 150% damage (130-160).