All of Galio's abilities have some trouble.
Resolute Smite [Q] has seemed to be dialed back in damage due to the AoE component and slow, but the missile speed makes it seem very swingy- people consistently dodge it or don't. The ability doesn't feel like it is very sneaky because you almost always want to toss it at maximum range.
- It has been a fun spell at times, but I agree with Don Heulnich that Galio lacks good reason to be in melee other than for Idol of Durand. I feel Resolute Smite could be replaced to add melee incentive somehow, though I don't have a current suggestion.
Bulwark [W] has cool interaction with his current passive in that it boosts Galio's AP by boosting his Magic Resist.
- The ability has a number of use cases, but putting it on an ally almost seems like it should apply the healing to them as well.
Righteous Gust [E] deals fairly consistent damage given its cast time and satisfying window for dodging the ability, but it's not especially threatening.
- It could maybe push enemies back at cast time- a small amount like Maokai's Arcane Smash (Q) or Diana's Moonfall (E). I think that would be very thematic.
Idol of Durand [R] has plenty of weaknesses and plenty of use cases. The only frustrating case I find while playing him is the set of on-hit stuns that knock him out of it- mostly Udyr's Bear Form.
- Perhaps he forms a protective shell during his ult that stops (some forms of?) cc but breaks down with each attack/cc proc.
Runic Skin [Passive] works well for an anti-mage tank.
- I know his passive has been mentioned by Riot as a point that should change. Presumably it makes Galio too focused against mages.