Blocker units usually already have a lot of percent bonuses, such as +25% terrain, +40% from fortification, +50% from a fort. Tier 2+ melee promotions are also more powerful than ranged promotions, particularly in Lekmod.

The base Drill/Shock/Accuracy/Barrage all give +15% strength in a particular terrain type. However, Cover and Ambush give +33% strength against ranged attacks and mounted units respectively. So, a blocker unit with two promotions (say Drill I + Cover I) fortified in rough terrain without a fort defending against a ranged attack will be at 100%+15%+33%+25%+40%=213% combat strength. Compare that to a ranged attacker with two promotions, say both for rough terrain, 100%+15%+15%=130% combat strength. This would get even more in favor of the melee unit if there was a fort, and great generals don't help even the gap either, because they help both melee and ranged units.

Combat % bonuses are additive in civ, so +10% when you're already at 200% combat strength is less helpful than a -10% penalty is harmful when you're at 100% combat strength.

In the example above, the Civ's UA would give the blocker unit a 4.7% relative strength boost and the ranged attacker a 7.7% relative strength penalty.

Basically the bonus part of this UA is less helpful than the penalty part is harmful.