I'll just tell it like I have told and spoken with others. Removing the unstoppable to Ornn's W does nothing about the issues many folk have with him, his damage. It takes away a form of skill expression with being able to tank hooks from the like of Nautilus or Blitz for his team, takes away tanking Zoe Sleep for an ally(assuming you apply it before the sleep proc and not on contact), and the biggest part I'd say.. Avoiding knock ups and displacements to get back in the fight faster than someone who would be. So let me just put what would be better nerfs I'd think from the time I've spent playing Ornn since his release. He quickly became one of my goto champs and my favored pick in the game, even with being relatively weak on release before the various buffs.(remember how he everyone considered him a troll pick back then?)
First off is the one addressed with this patch. His R cooldown was far too low for the power it has and honestly expected it to be nerfed sooner than this. Though now that it has, will need to see how it plays out now, but I think it should be fine.
Second, the Unstoppable aspect of W was perfectly fine. The flames are honestly fine as well, as they alone don't chunk(less someone is a potato and gets baked in the full duration). For people that say it should be current health, well, that's what it was before on his release and it did nothing unless the targets were full HP or near it.(part of the contribution to him being a troll pick on release until it changed) Low hp targets were practically safe around Ornn unless they got hit by what needs nerfs now. And that part which isn't fine and what should have been nerfed, instead of removing Unstoppable, is the Brittle auto attack. This is where a lot of Ornn's damage comes from. It's they key aspect of Ornn's kill pressure, especially post-6. If that bonus magic damage Ornn did when auto attacking a target with Brittle was nerfed, he'd be much less of a bully in lane and more in line with a CC team fighting tank as he's meant to be.
Third, and this is only if the Brittle AA's magic damage is lowered and he's still a problem, then his W should be looked at some more. Be it the base shield or HP ratio. Lower either of those or nerf the cooldown in the early ranks, but still returns to his live cooldown at rank 5 at least so he still has some pressure in late game fights.
For those that don't want to read.
Give back Unstoppable for skill expression among Ornn players. His R should already be relatively in line now with its nerf, need to see how it pans out for sure though.
Lower the Brittle AA magic damage that chunks. Sometimes it does even more than the flames depending on how much an enemy sits in the flames, not to mention you can get 2 procs in quick succession when paired with R.
If the Brittle AA nerf is not enough, lower the shielding(either base or the scaling) for more risk to Ornn or make his early W cooldowns longer duration. Not both at the same time. See how one goes then hit the other accordingly if still too strong.
If, for some reason, he is still an issue even after those have been hit or tweaked. The next best thing I could suggest to go with those are to raise his mana costs on W and possibly Q as well if needed.