I think conquerer is going to be most problematic on skirmishers rather than juggernauts like darius

The main reason being that conquerer is in the precision tree, which utilizes a lot of stats that juggernauts generally don't feel great getting compared to the entire tree that is resolve and even many parts of domination and sorcery.

For champions like irelia, jax, warwick, shyvana, etc. however, precision is great.

This is especially true with the true damage conversion which has diminishing returns for high armor pen champions like darius.

But you know who might be disgustingly oppressive with this rune? Ironically, this guy. He can build almost pure tank and output disgusting levels of damage right at the beginning of a fight with his Q and R even without his E. Converting 20% of siphoning strike to true damage might honestly just break the ever-living shit out of him. And remember, general combat is the only requirement for readying the proc, not combat with specific champions. Darius will pretty much never have the ability to proc conquerer and noxian might at the same time, most likely, darius will proc conquerer and then have to stack his way up to might to really make great use of conquerer. Nasus just needs to hit the minions a couple time, and then when he goes to proc conquerer, can press R and go from 0-100 in less than a second after proccing conquerer. The AD waste won't matter, the true damage conversion for spammable siphon strikes will be the real gold.

There's nothing wrong with being scared of a champion like darius or illaoi (cleaver build) with conquerer, but you may be surprised at what the real abuse cases lurking in the shadows will be capable of.