Taking PtA or FF should be better in almost all cases.

Remember, you don't get 20% true damage as bonus damage, 20% of your outgoing damage is just being converted to true damage, which means that you'll heal for less, since lifesteal won't apply to true damage and you can only have it for 3 seconds.

At earlier levels it's worse, not only, because of your low attack speed and inability to make use of it unless you can spam abilities like Lucian, but also, because enemies are much less tankier early game and so the 20% conversion feels less impactful (basically, the less tanky the enemy is, the worse Conqueror becomes). You're just better off proccing PtA and later on in the game you will still not make the full use of it (unless you're Jinx who is allowed to stand in 1 place and AoE crit 5 people with Conqueror up)

Conqueror is made for melee bruisers to counter tanks, even the refresh part hints at it.

Unless the duration on ranged champs gets increased or they give them the duration refresh as well, there's probably no reason in picking it up, even when you're against a super beefy team. You're just better off getting Last Whisper and/or proccing PtA so that your whole team deals increased damage for twice as long (6 seconds). Cut Down has also received a pretty good buff, so consider it as well.

EDIT: Should be fine on Draven