It's not just ADCs (though they are a heck of a contributor to the problem).
It's more the fact that it's super super easy for a group of three or more people to keep him far enough from them where he's not a threat.
He's big and slow unless he itemizes movement speed among his tankiness (which I've done on a number of occasions and it was hilarious, especially with Talisman), and the only thing he can do at a distance is slow/cripple one person (but a hell of a slow), and drop a stationary AoE.
I mean, I'm not complaining that that's a really good way to nullify his strengths as it's an intended weakness, so fair enough; these days it's hard to feel like the payoff for managing it is justified, though.
And heaven help you if you rely on pulling a couple from the enemy team to deal with you, because they'll likely ignore you, tear through your team 4v5, and take objectives faster than you can.
But yeah, the 'late game monster' thing for him hasn't been true for several years now (outside of very niche circumstances), and it irks me whenever folks yell about his infinite scaling when it's got a very narrow method of employing that power. All the stacks in the world mean jack if you can't get close enough to actually hit something with them.