Galio is a good champ, but the problem directly relies in his ult.
In a meta where champs with on-hit AA CC effects like
and
are popular, his ult is nothing more than a 0.5 second taunt that's immediately cancelled out. Even then, almost every champion has a CC ability that outranges his R radius that can cancel it.
A few days ago I even played a game with Galio mid. I used my ult about 10 times that game, and NOT ONCE did I have a full length ult.
Besides
's ult, it's probably one of the hardest to pull off ultimates in the game. Of course it has the potential to be game changing, but it's High Risk, usually Low reward. As a mid laner with 3 damaging abilites, having 2 of them as skill shots and the other as very unreliable, there is no incentive to play him over someone much safer like Annie, who is actually very much similar to him but scales better as well. He'll remain a niche pick for those times that your facing 3 APs to me.
Imo, he should get a minor rework or a few tweaks that center him around jungle. I've tried it a couple of times and the sustain from W is not bad at all and ganks are almost always certain kill. It's either that or buff his AP ratios. Hell his Q got ninja nerfed by 0.1 AP one patch for absolutely no reason, he's been neglected for almost ever.