The Thing with the Ruined King
Or, rather, his Blade: 
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As anyone who follows the Boards' discussions can attest, Blade of the Ruined King has been balanced on the blade's edge for the whole of its existance. Let's dive into the reasons for that:
First, the central purpose of the item: At its core, Blade serves as a tool to counter HP stacking on enemy champions. Its intended users are champions that auto-attack the enemy alot - a classification that includes ranged AD carries, but also champions that are, nowadays, classified as divers, juggernauts and skirmishers like 
(not a comprehensive list by any means).
This diverse user group, by extension, means that BotRK is supposed to serve groups of champions that have, essentially, schizophrenic demands on the item and its statline - in addition to the demands on the statline its intended purpose places:
- For most of the ranged ADCs and some Skirmishers, Blade needs to work well in the context of a build with multiple scalars and a relatively rigid structure with a least 4 interacting offensive items (Infinity Edge, Multiplier Item (nowadays, a Zeal item), Lifesteal item, Last Whisper Upgrade (in a game that warrants anti-health stacking, anti-Armor-stacking is virtually garenteed to be required as well)).
- For most of the Divers and Juggernauts, Blade needs to work in a build that has, at most, one other dedicated offensive item. This means that, for these champions, it would be best as a relatively balanced (in terms of stats, not game health) item that works well on its own.
- The fact that its intended anti-HP-stacking effect is conveyed by means of an on-hit effect strongly suggests that the item has a statline that amplifies the frequency of auto-attacks - meaning, attack speed. However, attack speed is inherently a multiplier stat.
- Another effect that helps the intended countering of HP stacking is Lifesteal, more specifically Lifesteal that works with the HP-attacking on-hit effect. As the health return from Lifesteal is based on the actual damage dealt, having high EHP against physical damage by means of high general HP and low Armor makes the HP stacker, essentially, a health battery for the Lifesteal user.
Does everyone see the potential for conflict in these demands? To satisfy the first demand, to work on the offensive auto-attackers, it needs to be able to be slotted into the either the Multiplier role or the Lifesteal item role. To work as a multiplier, it would need Crit Chance - but having Crit Chance essentially bars it from the hands of the Divers and Juggernauts, as Crit Chance is not really that great if your build only has a single Crit item. That leaves the Lifesteal role for the item - which, for reasons outline above, works well against HP stacking. But, for Lifesteal to work with the on-hit effect without being a nightmare from a coding and intuitivity standpoint, not to mention from the synergy with the above builds, the on-hit effect needs to be physical damage.
To be good for the second group of champions, it needs to be good on its own, as a single offensive item (or part of a two-item combination). As it needs Lifesteal to work on the first group of champions, it will also have AD - the two stats are tied inseperably together since the pre-season 3 item overhaul. However, an issue with having just Lifesteal and AD on an item is that it would not make an item that works well on its own on an auto-attack based champion - especially since Blade also has the anti-HP on-hit effect. That leaves attack speed as a stat that both groups of champions are interested in. Furthermore, the second group of users is also less interested in the Lifesteal of the item - they won't complain about having it, especially not in the laning phase, but it is nothing they'd specifically buy the item for. As these users build quite tanky in addition to their one or two offensive items, they are not nearly as reliant as the first group of users on the health return effect to take the tank down.
And now we throw an unexpected curveball in: The item is also supposed to have an easy build path (ok, that's doable) and build out of Bilgewater Cutlass (wait, what?). Building out of Cutlass means that it needs to inherit Cutlass' active effect, as well - which is a little burst of damage and a slow. Having the item be balanced and desirable for all user groups means that the active needs to be kept as uneventful as possible - the more of the power budget it consumes, the less is there to balance the statline for all users. *
So, does everyone now see how these demands could result in a problematic item? Have too much power in its synergistic effects (i.e., the Lifesteal and the on-hit effect) over the direct offensive stats and it becomes less useful for the group of users that want it as a single offensive item (this was Blade way back, when its stats were just AD and Lifesteal without attack speed). Too much power into the active, and it becomes an item that is mainly used as a nuke on a burster (remember Zed7Twitch/Vayne in S3?). Too much power into the direct offensive stats, and it will be too good as a generic single offensive item (we did not have this situation yet, fortunately. Just imagine a Vi with an item that has 55 AD and 40% AS).
Currently, we have the situation that Blade has a lot of power in both its direct offensive stats and a bit too much power in its synergistic effects. This makes it very strong on the first group of its intended users - it is a viable solo offensive item, but also a very good platform to build other offensive itemization on. This is emphasized and most obvious on the ranged AD carries, due to them both lacking a generally self-sufficient single item buy otherwise and due to the regular melee/ranged dichotomy.
The upcoming nerf to its Lifesteal will take power out of its synergistic effects, disproportionally affecting the first group of users. But, unlike the previous nerfs that lowered the on-hit's strength on a significant part of its intended users, the Lifesteal nerfs will not weaken the central purpose of the item - its intended role as a counter to HP stacking.
- Side note:
For the longest time, the strength of the active was a big part of the issue with Blade. Since they tackled that, its stats should be way easier to balance.
Remember, for the longest part of Season 6, the power in the active kept and the resultant nerfs on every else, especially its main selling point of being anti-HP stacking, kept it in the rarely used part of the item pile - and its synergising users in the rarely used part of the champion pile.
