How is his W far more fitting for a bruiser?
This
and this
and you are set.
Your Q applies 2 stacks of Cleaver and with Visage you heal more damage than you take.
His W is reactive, assassins aren't reactive. They are aggressive, they will make the first step or anticipate what its coming.
Qss is perfectly fine.
Which means that after you used your W you will get destroyed by the next CC. Rengar escape is shit compared to other assassins, so he will be forced to build tanky if he doesn't want to get destroyed.
If Rengar was working was due to his EMP W and his stupid damage.
More reasons?
His Q has a cast time that makes him stop and being stuck during its animation, Assassins don't want to being stopped.
His Passive allows him to build not many damage and still have ton of damage. Also you have to spend around 2 seconds building ferocity, an Assassin doesn't want to spend sooo much time for killing the target.
They want to: Go in->Do damage->Out
Not: Go in->Stack your damage->Damage->Out This fits a bruiser more than an assassin.
A typical bruiser Rengar build will give you:
Around 320 AD
Around 120 of Armor and MR.
Around 3k Health.
That's stupid AF.
but there is a lot more counterplay
Another problem for assassin Rengar.
You struggle reaching your target due to your ult indicator, and after that you will struggle (if you don't get the crit, which shouldn't exist at first place) at killing it because your Q has a cast time, if you fail it you are dead, and after that you will struggle at going out since your escape isn't really good.
However if you build bruiser:
You will struggle reaching your target, you won't struggle at killing it because the room for errors is higher, finally, you will get out easily due to your tankiness.
just going for empowered Q.
Not true.
You used EMP Q for safe and secure damage if you were sure that the target would die.
You used EMP W if the fight wasn't going good and you wanted to restore some heal back.
You used EMP E if the target had a lot of mobility, also this was the safest option but it wouldn't oneshot the target since the damage was lower.
Now, however (talking from 7.2 perspective):
-Emp Q is only useful for a secure kill (Not really changed). However most of the time you have enough damage without this skill.
-Emp W is pretty much the best option since you will avoid all CC and will help you escape and reaching your target.
-Emp E lacks of use since Emp Q has a dash and Emp W avoids CC. You only use this for disengage or trap a target for your team.