My beef with Conqueror is that there is already plenty of ways to penetrate armor 
Black Cleaver is already huge as it REDUCES armor -- so everyone on your team benefits from it, combined with a and the tank has effectively lost approximately half of their armor from just two items. Combined with most assassins building lethality and tanks can easily have less than 100 armor unless they're building nothing but full armor. This is not even considering champions who already have very good abilities to shred through armor in the first place.
But now, add 20% true damage on top of that with an additional 15~35 AD just because. And since you're already doing a lot of AD from all of the shred, the true damage is higher. It's not like it has a severe requirement for activating, either.
I've literally seen Ezreals run Conqueror (and ignite) and have around 30K true damage to champions in a match. It's absurd. Trynameres and Master Yis do even more than that. Yi's do a LOT more than that, actually. Sometimes 70% of the damage I take from Yi is literally just true damage.
My main, biggest gripe with Conqueror though is that it's going to snuff out creative design if it sticks around.
Champions like Yi and Trynamere (Trynamere in my opinion has an unhealthy kit imo anyways) doing half of their damage as true damage without even building armor pen (more than half in Yi's case) just from building I.E. and having a passive is not healthy design especially when it can be quite difficult to stop them in the first place and true damage is very difficult to itemize against because health alone means nothing when an enemy is still slapping you for over 1K damage. It can take over 3,000 gold to just survive one extra hit, which when they can easily attack more than 2 times a second, isn't much at all and armor doesn't do very much either because Black Cleaver and Dominik's exists to already make half of their armor value moot. And again, this is assuming there isn't even a champion on their team that doesn't even have some form of innate shred in the first place.
Not to mention with Yi, items like Thornmail or Frozen Heart are already somewhat ineffective against him since he doesn't suffer from the attack speed slow.
Finally, conqueror itself in my opinion ignores interactivity in the game. Sometimes you don't even build armor pen despite the foe having lots of armor because you can just build damage and it'll work out because hey, free 20% true damage on top of a generous AD steroid.
In a meta where tanks aren't getting literally nuked in 2 seconds and true damage wasn't already abundant, conqueror may work in some form, but currently it is doing nothing but contributing to a problem that people have been literally quitting over. I've been playing League since open beta. I've seen tanks be weak before, but I've never have I've seen a full tank melt so quickly to just one opponent by just auto attacking them to death as I've seen this season.
I remember when true damage was quite small. Things like Twitch's passive, red buff, etc, all did single to double digits of true damage, aside from ignite or smiting a monster. Now-a-days it's common to see true damage in the 4 digits in mere seconds or even from a single ability, and tank items do not grant nearly enough health to combat that amount of true damage.
People say "just build health!" If you build nothing but health though, you're going to nullify the true damage but you're going to still die very quickly to anything that's not true damage and you're VERY easily going to die that anything that damages you by % health, or damage based on missing health -- which is another problem in itself as there's also a lot of % true damage or % missing health true damage that there's also no way to counter and are built directly into many champion's kits.
's in a gutted/unplayable state.
w, 
ult, and 
e get dumpstered. idk how it interacts with 
though
