People say Conqueror is the weakest rune
The problem with Conqueror is that it does not serve its intended purpose and it's the most abusive rune. It only allows those who can abuse it well to be viable and removes the point of getting bruisers to be viable. It's a split-pushing rune with little interaction.
Riot keeps saying "we want interaction, we want interaction".
Then what's the hesitation on Fervor? It REQUIRES you to constantly engage with a champion for extended trades to get its full potential. Conqueror requires you to be useless and AFK or being in the lead for 4 seconds before working.
It's weak because it's a rune that isn't useful for most people while also too abusive on a few select champions. It's toxic in design. It should be removed as a whole and just replaced with a reiteration of Fervor.