My unbiased opinion on Duskblade (been playing since Season 2, high opinion value)

For an item that is meant to execute, it's honestly terrible at it. I rarely get kills with the passive. The base stats are honestly the real reason to buy the item. Not the passive.

This is because Riot put a lot of the item's strength budget into its base stats instead of the passive because the passive is 1. Highly unreliable and 2. Has a huge cooldown.

Riot has, essentially, backed themselves up against a wall by creating this item. Because if they buff the passive too much, it'll be easily overpowered. It's not an easily balanceable passive. But it's weak right now. They try to make up for the passive's weakness by giving the item good base stats, but there are other items with 75 AD that give you far more useful passives and actives, like Bloodthirster and Hydra.

Duskblade is still a good item on Zed; another way to proc Thunderlord's and helps secure ult kills. It's also fun to ult one enemy, auto another enemy, and watch them both pop at the same time. But its damage is still pitiful, and isn't quite as useful on other champions.

You don't need to be a game designer to see the game design issues with the item.