What..? The new LW only affects Bonus Armor... And you'll max at 47% with it.
You're looking at the wrong half. +15% damage against high HP champs.
Any champ who builds armour is going to have at least one HP item, meaning you're going to have +500 HP on them and they get 15% bonus damage against you.
I've played this out on the PBE from both sides. I can tell you tanks are incapable of tanking on the PBE. The 3 ADC comps will melt you in a second flat even with 300+ armour. ADCs are so strong on the PBE that they don't need BotRK to melt tanks. (not that they ever did, IE's always hurt tanks more)
Merc Scimitar is ludicrous, it basically allows ADCs to completely ignore tanks, the MR and lifesteal make their damage irrelevant and the CC cleanse means you can't CC them down.
Lord Dominik's Regards punishes building every single tank item in the game. You lose 40% of the value from any armour you buy and any HP you buy makes you take more damage from the ADC. To make it even worse the components for this item are also incredibly powerful. You can just sit on you 1300 30% armour pen item, you kind of just buy it when you have gold and as long as the enemy has one armour item it's incredibly efficient, same goes for Giant Slayer but for HP instead of armour.
Also you mentioned masteries, have you seen how crazy the ADC ones are? Sure tanks have a decent one, 8% damage reduction is nothing to laugh at, but it's honestly not as good as two of the ADC masteries. Warlords Bloodlust is utterly absurd, heal for 15% of a crits damage and gain 20% attack speed, I think it's honestly the strongest mastery right now. Slightly less talked about and a bit more situational but Fervour for Battle can get insane. When it's fully stacked it deals 80 on hit Phys damage to each auto. If you keep this stacked it's greater than the 8.8ish% bonus damage you need to overcome 8% resist.