I feel like Galio is in a place where he has too many trade-offs in the decisions he has to make, where so many champs now days don't have quite so many trade-offs, or none quite as extreme.
Built full-tank, he can definitely absorb damage as well as the best of them. With a Visage, the heal from his W is nothing to laugh at. Unfortunately, the damage he puts out as a tank is extremely lackluster. And his utility is very limited and his ult, which already has a pretty high CD, often feels gated by having flash as well. Between the limited utility and the low damage, he really isn't much of a threat at all if his ult is down.
Then there's the mage build, which I personally feel is really his best option right now. With AP, he actually has extremely fast wave clear at a decent range. The MR on Athene's makes it just that much more efficient for him. And Abyssal becomes a core item as well. And since the heal on his W scales with AP, he can actually absorb a fair bit of damage while it's on as well. Of course, this build is also not without glaring weaknesses. For one, the Q projectile is slow. It has a decent-sized AOE, but honestly it's not the easiest skill in the game to land. The Gust is easier to land, but the damage is pretty mediocre. Then there's the ult. It does quite a lot of damage, which is great. If you can actually pull it off. But again, the effectiveness of that ult often feels gated more by the cooldown of flash than the ult's CD itself. And even when you do get a good ult off, there are too many things that can instantly stop it now.
It basically all boils down to him being very unreliable for his damage, which is accentuated if you try the mage build. At least with the mage build, you can deal solid poke or offer wave clear with his other spells, unlike tank Galio. And mage Galio actually can draw focus of the other team better than tank Galio can, so in that sense, he probably tanks better as a mage than he does as a tank.
