While I've been working on the item comparisons for Master Yi, I've gotten some concrete calculations on Blade of the Ruined King to back up some things I'd like to mention about Bork.
Disclaimer: I know that Bork is amazing, and as the active is difficult to model it's not included in calculations for sustained damage. I just wish people knew what they were giving up on when they buy it, rather than just crying OP.
Firstly, the good stuff. It might be the single most cost effective item late game. At level 16, for Yi, it was almost 30% more effective than the second most effective combination of two items, which by the way cost almost twice as much. And that's without even talking about the life steal or the active.
However, there's more to the story. First, I'd like to point out that Master Yi almost only auto attacks, so he'd be a good example of someone that can use all of Bork's stats.
Lets start at the start. The rushed Bork is very cost inefficient. It's less cost efficient than Ghostblade (even assuming the active is only relevant half the time), Infinity Edge, BF sword, Phantom Dancer, hell, even a rushed Last Whisper is more cost efficient, even at average no armor item values. It's only a petty 6% more efficient than ninja flippin' longswords.
Another thing it's been praised for is the build path. But again, it comes with sacrifice. (First and foremost, on AD casters the two daggers are almost entirely wasted, but back to Yi). The cutlass that's included in the build is extremely cost inefficient unless the active and life steal are absolutely amazing in your situation. It's beat out by all items in its cost range, and most below it. BF sword, brutalizer, zerker greaves, longswords and doran's blades, stingers and zeals, even daggers, spirit of the elder lizard and executioner's calling are more cost effective. And by quite a margin, the Cutlass is literally less than half as effective as a BF sword. The only item it beats? Avarice Blade.
But it's amazing super lategame when everyone has bunches of HP, right? Nope. In a classic IE PD LW build, adding a BT is better than adding a Bork - even if the target has a Warmog's and a Sunfire for extra hp. And that's even assuming only one Alpha Strike target.
Here's the thing. It doesn't scale. It doesn't scale well at all. It has such poor synergy with almost everything. It's very low AD makes it not help your abilities. A ton of its damage comes from the % current hp, which doesn't crit or do anything with your AD.
So most importantly for auto attackers, it doesn't crit well. And with that I don't just mean build critless, if you go critless you're missing out on a major multiplication axes, which even comes with its own scaling stat - crit damage.
Most important for AD casters, it doesn't scale your abilities well, and few AD casters get to auto attack a lot, so the ASPD and %dmg is mostly wasted.
With that result, returning to my first statement about its efficiency, looking at Yi, comparing over 50 different builds at various levels and price ranges, it was never a top contender. It was never even top 5, in any of the builds. In fact, most of the builds including Bork are sorted neatly at the bottom.
So, in conclusion, when is Bork good?
It's good starting sometime during the midgame (earlier if they build hp) when you're not using other items for damage, when you're auto attacking most of the time and when you don't have good AD ratios. So mostly bruisers, think Irelia.
It's good when the active is really good for your situation, and I don't just mean the damage as that is quickly outpaced. For a split pushing dueling Zed, it's amazing, having synergy with not only your ult damage amplification, allowing the damage to scale on another axis, but also your strong auto attacks that it lets you apply much more freely compared to its main contender, The Bloodthirster.
Another of it's strengths is that it contains both AD / on-hit physical damage and attack speed. Carries like Vayne desperately need both AD and ASPD due how her kit works.