Most of it is coming from those who can't do math. The rest is probably misattribution of failure to Conqueror, rather than a lack of skill or the character using it.
Conqueror is 6 AD, or a Stat Shard at Level 1. At Level 3, when junglers are showing up, it's at 9.41 bonus AD. It's giving roughly 12 AD at Levels 4 and 5, when experience starts to slow down, and if they're going to all in you at 6, it's 14.53 bonus AD. If your complaint is they get a free Longsword and a half at Level 6 in exchange for making their plays around keeping stacks up, that's silly. You would lose less farm backing off, giving up cs, and focusing on forcing them to drop their stacks or take harass, than you would to playing a melee into a Teemo. If you can't do that, it's not the rune's fault.
At Level 13 Conqueror gives a Pickaxe worth of AD. I haven't seen anyone complain about Conqueror's bonus AD in mid/late game, oddly, even though that's probably where Conqueror has the highest gold value in a vacuum, relative to other runes.
You're right about damage conversion. The problem is, nobody understands how armor works.
Armor is a damage multiplier that is, usually, less than 1.0. Specifically, if your armor is > 0, Physical Damage = Physical Damage * (100 / (100 + Armor)) If you learned how to work with fractions, you might notice that if Armor > 0, the expression 100 + Armor will always be greater than 100, and so the denominator will always be greater.
Now, recall what we've learned about multiplying by numbers such that 1 > x > 0
Base Armor ranges from 17 (Orianna) to 47 (Braum, Leona, Tahm Kench). Armor at Level 18 ranges from 68 (Orianna) to 130 (Pyke), not counting Thresh because he gets armor from souls, not levels. ADC itemization does not currently include Armor. Mages almost never buy Tabis because it gimps their damage mid/early, and usually buy Zhonya's Hourglass, so you can add 45 to any of their armor values to calculate in mid/late game. Supports typically buy Locket of the Iron Solari (30 Armor) or Zeke's Convergence (60 Armor), so add that as appropriate.
TL;DR In other words, unless you're attacking another Fighter or a Tank, you will never be dealing with more than 200 Armor.
So let's go back to our Armor formula.
At 51.6 armor, or that Level 9 Caitlyn staring at your bot tower licking her lips, we're looking at Physical Damage * (100 / (151.6)) or Physical Damage * .66 Caitlyn's reducing 1/3 of your damage. That's a lot! Hold off on those ADC in 2019 memes!
Let's head mid instead. Kill that OP Cheese Leblanc while Hourglass is on cooldown. Level 10 Leblanc has 49 + 45 Armor with that Hourglass. Let's use the formula; Physical Damage * (100 / (194)) or Physical Damage * .52 Wait, so Leblanc can ignore just under HALF your damage? WTF silver balance team how is she even in the game???
Level 10 Maokai is pretty braindead, let's go ruin his day. He's got his Sunfire Cape (60 Armor), Stat Shards (6 Armor), Aftershock (96 Armor) and he gets armor just for being Maokai and Level 10 (70). That's a whopping 232 Armor. Physical Damage * (100 / (332)) or Physical Damage * 30 and I don't need to explain why 70% damage reduction is a big deal.
OK! So now we know how Armor works. Let's take our 3 realistic targets and see how Conqueror affects all of them.
We can represent this by modifying our formula. For brevity's sake, let X = Physical Damage. This way, we can mathematically represent True Damage as Physical while still accurately showing what happens to their health bar. Let's call our 20% True Damage .2 * X, OK? Armor's only going to apply to the other 80% of X.
Damage = (.2 * X) + (X * .8 * (100 / (100 + Armor))
See, all we've done is use math properties to apply the armor formula only to part of X. Now let's test those old numbers and compare the difference in damage reduction. (We won't be able to factor in the bonus AD, because that matters differently based on AD ratios and specific kits. But Conqueror is giving 21.35 AD right now, at Level 10)
vs 51.6 Armor: Damage = (.2 * X) + (X * .8 * (100 / (151.6)) = .73, previously .66
vs 94 Armor: Damage = (.2 * X) + (X * .8 * (100 / (194)) = .61, previously .52
vs 232 Armor: Damage = (.2 * X) + (X * .8 * (100 / (332)) = .44, previously .30
So you're getting 7% more damage against the ADC target, 9% more damage against the mid laner with hourglass target, and 14% more damage against the top lane tank with Sunfire and defensive runes. Compare with Press the Attack which is giving 10.12% increased damage at Level 10 to the same bruiser taking Conqueror. Press the Attack is also giving that bonus to your teammates.
I cannot stress this enough. We just used math and realistic examples of stuff you'll see in game to show that without factoring in the bonus AD, Conqueror is strictly worse than Press the Attack EXCEPT against targets with over ~200 armor.
and people read "The bonus true damage can doubles the damage that procs it," but ignored both the cap on the damage as well as the fact that, if it dealt magic damage, the MR of the target would essentially be worth more when protecting from the second trouble bubble proc.
