Am I missing something? Bruiser Rengar vs Olaf

I wana make this very clear: These are 2 of my top 5 favorite champions in the game so I am trying to be as unbiased as possible about this post.

my argument Bruiser Rengar can effectively do his job EASIER and possibly better than Olaf. I could write a wall of text comparing these 2 but i will try to keep it brief. is the build i use, and ironically it is a solid build for Olaf too. Now my issue or rather complaint is that, Rengar who is meant to excel at blowing up the adc or apc, is also outshining (imho) Olaf at being a bruiser too. Rengar can stealth get MS from items/ult active and run in, jump directly on his target and hold them in place, while Olaf, although being immune to CC, loses his passive resistances (rengar actually gets to use W to get bonus res, which makes his HP go further in these bruiser items) and then has to muscle his way through enemy champions throwing slows at his target but never actually being able to completely hold them still like that of rengar.

So why is it that Olaf has to sacrifice his tankiness to get to the back line while rengar doesn't. There has to be something i am missing here. I suppose Olaf could make a better full tank than rengar (maybe) but why is it that rengars 2ndary role seems to be just as strong if not stronger than olafs primary role?

TL;DR Rengar can get to the back line easier than Olaf and hold his target down better which reduces the amount of chasing rengar needs to do compared to Olaf. While also keeping his tankiness and not sacrificing damage the way Olaf would when trying to itemize.

I admit I've only done this bruiser build on rengar a hand full of times so I'm not sold on it yet, however, in the games i have played with it, i can see a fair amount of strengths and weaknesses, and can honestly say its quite viable.

Some might disagree with me, but i feel Olaf could use a slight buff.. Possibly a little more slow on his Q to help him stick to his target late game without needing a bunch of freaking items to assist him with it. Or, revisit the numbers on his ult and how they work, such as keeping the resistances even after he activates it. Or increase the resistances when he ults and reduce the AD. OR keep his theme, remove the resistances but increase the AD. Either way, i feel that Olaf has the short end of the stick when it comes to doing his job. Maybe i need to go back to building thornmail despite the nerfs to it.

I encourage you guys to try this rengar build, i find it pretty fun. Its a bit more team reliant as you jump in snare your target then beat the shit ouf of them with your team. The nice thing about this build is you can actually initiate w.o getting blown up, and still deal respectable damage. Late game, after picking up the titanic, I was bursting down ADCs nearly as quickly as i was with an assassin build. =]