Percentile damage is not that gamebreaking. Vayne can deal the same percent of damage after 3 hits, make it true damage instead of magical, and make it based on max health, not current health. She can then buy a Blade of the Ruined King, deal that damage every single strike as physical, and get 10% of max health on her active on top of it.
That all is of course based on base damages, which can be improved upon with AP. However, with the meta build not even focusing AP, I rarely even recognize it. Even if I did, she'll have to tank quite a bit regardless, and if she does that, might get the near 300 AP required to double that to 16%. Fat chance though, methinks, as she really needs her tankiness to contribute at all in teamfights. Meanwhile, Vayne can still do quite a bit of percentile damage with a stun of her own, a movement speed boost, a dash-type skill, and possible invisibility to boot. Elise simply doesn't have the utility to pull off a simple percentile kit with her mostly melee kit. She does have other tools, I'd say, but percentile means jack if you can't execute it.