Quite frankly, he' just inferior after nerfs after nerfs after indirect nerfs after indirect nerfs.
There was the first set of pre-emptive nerfs when Riot stupidly thought he'd be OP just because the meta started seeing dual AP long LONG ago.
After that, it's been nothing but one ufcking indirect nerf after another.
MR rune nerfs = &U@%'s Galio harder than anyone else
MR itemization nerfs = same as above
The rework to Penetration = MASSIVE nerf to all tanks
REMOVAL of several expensive MR items = Galio gets focked
Repeated nerfs to Athene's = Galio's core item nerfed repeatedly, while all champions that uses it is considered nerfed for it, Galio is hit harder
Riot constantly adding more and MORE ways for enemy to easily interrupt his R with each rework and new champion
Recent mana itemization nerfs = yeah
In the end...Galio just received thousands of nerfs. Yeah, many of them hit other champions too. But one thing to remember here is that all those nerfs...hits Galio WAY harder than it his all other champions.
And that is all atop the unnecessary nerf he first directly received.
Not only is he a tank not named Mundo or Maokai, he's had way more indirect nerfs than any other champion in the game.