I want to stay neutral here as an Ornn player.
But it's hard to.
Ornn himself has never been the problem, even after all his severe nerfs/reworks. I personally enjoyed the unstoppable status.
But..... I'm doing fine without it.
I also don't build him like everyone else either.
But I beat them by 30%, pulling closer to 77% rather than 47% too.
The problem with Ornn is that people who play him haven't actually done the math on how certain runes work with him and choose the right runes, and also potentially build the wrong items.
AKA:
Learn the right items.
Learn the right runes.
I can beat nearly every single conceived "Ornn Counter" with Ornn. This includes:

I'll give you a hint:
AP is not the solution.
Armor is not the solution.
Resolve isn't as important as its Tier 3 and 4 runes on Ornn. You can other Tree Primary Rune him. In Fact, it's almost guaranteed to be better.
Knowing which opponents do what and when NOT to use R.
(If you R a
who hasn't R'd yet, you're not very smart).
Knowing not to use E as an engage unless it guarantees a kill.
Using Q to more functionally than W or E.
Personally, I'd love to have Unstoppable back. It would mean I could do more useful things more actively. But I do well enough being passive. (I do miss W through Veigar's Wall or ignoring Blitz's pull).