I was actually kind of sad that Entropy hadn't made its way into Twisted Treeline. It seems like such a solid item, and I like the active giving me freedom from RNG. I built it on anyone who needed the slow from Phage but needed AD more than the myriad of stats found on Trinity Force.
Pantheon (I know Mallet and TF are still solid, but that ratio on his Heartseeker)
-and a few others.
As for Kitae's, I like it in concept. It deals with tanky enemies.
Except, the majority of people who build it do physical damage, but do not have access to Spell Penetration. With a proper hybrid penetration item, it could do better, but as is, we're paying for 2.5% of their max health per hit, which is closer to 1.5% when you factor the innate magic resistance.
Then you add in the myriad of attack slowing items and abilities. Warden's Mail was such a huge/inexpensive counter to this item, especially when upgraded and/or paired with the number of MR items that bruisers gravitate towards (Wit's End and Hexdrinker/Maw).
So in essence, it defeats itself. You build it do deal with tanky bruisers, but you lack the means to optimize and fully benefit from it. They can counter the item with their natural build, and the investment to nullify it is cheaper than the investment to obtain it.
It wasn't worth building until the enemy had 2-3000 health, and even then, they'd have defenses and their own items to make it sub-optimal even then.