I will say, that shield is about 650 roughly. He has a Pretty big 80% ap scaling on it. Along side his mastery/Runes(which Buffs his Shield, And The items that also bump up the shield power. Thats a pretty fair amount.
He also is pretty weak in early games, and can be easily abused in that stage. However you had a Kayle jung who has an even worse early game(stronger late). So he had a free time Farming and getting items.
However. His Power budget is soley sunk in that ability. And the rest are pretty lack luster. He has no damage, and cannot secure most objectives when contesting. His CC is pretty Poor, Slow, and predictable. His Pseudo Invisibility lost most of its power with the removal of hiding objectives. His enhance autos don't really do anything aside from just being ranged.
I don't think Ivern needs nerfs in anyway. In a game where Damage is running wild, I think a champion that gives good shields is a good power to have in this meta. Its not fixing the damage, but it does work against it.
I would say IF damage is heavily nerfed from items and champions. The shields and heals are busted beyond. But with the amount of damage output there is. I cannot say the nerf to his shield is needed.
As for riven. That is such a strange comparison to make. Ivern is a support Jung while riven is a diver Top.
ivern is just supposed to help carry the carry. And Riven is a carry who can easily 1 shot 3 from full HP because she hopped or flashed a wall.
The frustration of riven comes from the Constant win/Pick rates she gets at higher levels Leaving her untouched for about 2 seasons? and IF there is a nerf there is a compensation buff to follow. Who can Build CDR While getting Huge damage, Life steal, Incredibly Mobile to catch and escape. Being a big Lane Bully(But not so much that it over blows even the overpowered But enough to reign over seasons.) With great sustain with the fact that she has no Resources. Riven isn't broken. But She is incredibly frustrating, strong and picked often.
Ivern Robs you of a Kill
Riven robs you of a game.