I commented earlier on a point similar, I think Galio's ultimate should have a mini-silence before it triggers, stopping prompted skills, but anyone outside the ulti can go in and cancel it, and enemies like udyr and braum still affect it regardless.
But even without that, I have to add that many times I have flashed ulti when blitz or leona have not prompted their stuns, but in that window (less than a second) before the ultimate stops these skills, they get activated. I agree with some comments, just having Galio present makes the enemy team very cautious with their CC and I have won 3 consecutive teamfights without using his ultimate, just by charging in and scaring the enemy out of position. I love him and how he can be played, but every once in a while I get this bug where I ulti, it gets cancelled before it triggers, and I get locked into the ultimate posture but without casting it. I just think stopping enemies from just tapping that AA CC "As" they get taunted is unfair, they should either have it primed before hand, or they be outside and activate it, so somehow closing the window where they can counter it would be nice.
But to be fair, something much simpler would make Galio's life easier, if his ultimate is cancelled, reduce the cd of the spell, not a lot, but from 3 minutes to 2, most of his opponents have such short ultimate cd that even though he counters them, if they are just aggressive enough they can hit their ultimates twice before he can use one that is very difficult to land or get in range with
Shorter cd if its cancelled, and turn his e into a charge and his spells would synergize much better without taking away counter play options