First off: it's not a shield. Shields are that gray/purple/orange things that appear on the end of your health bar. He has a defensive steroid coupled with a heal.
Compare his defensive steroid with Rammus' and you will see how it's not that great.
Galio's Bulwark lasts for 4 second and grants him 90 Armor and MR. It costs 60 mana and has a 13 seconds CD.
Rammus' Defensive Ball Curl lasts for 6 seconds and grants him 120 Armor and MR. It costs 40 mana and has a 14 seconds CD.
In other words: Rammus' defensive steroid lasts 33% more, grants 33% more stats, costs 33% less mana and only has a 1 second longer CD.
For further comparasion:
Graves' E grants him up to 120, which can be refreshed indefinitely.
Malphite's W permanently grants him 35% extra armor. And this includes his base armor.
Nasus gets up to 100 Armor over 15 seconds during his ult (apart for the bonus health).
Singed gains 80 Armor for 25 seconds.
Trundle steals 40% of his target's armor during 8 seconds.
Xin Zhao gets up to 125 Armor with his ult for 8 seconds.
So yeah, it's not a bad steroid, but it's far from "pretty much makes him take 10 damage from everything". REALLY far.
His mobility boost can be rendered useless by the enemy simply walking into an diagonal patch in relation to the vector of the ability. Not that hard.
His "3 damage abilities" include a slow moving projectile that deals considerable damage, an almost ignorable straight line skill shot that is maxed as a 2nd or 3rd ability, at a time that nobody really cares about it's small amount of damage, and an ultimate that is honestly VERY strong and deals considerable amounts of damage, but also require him to be in almost melee range and forces him to take lots of damage.
His passive grants him about 189 AP with the following build
. This build means that he is giving up on having any meaninful amounts of Armor and HP. Coupled with his scaling, those 189 AP will grant him an extra 400 damage if he lands his QER (with maximum stack on his ult). It's not that meaninful, but it is indeed a nice addition.
Galio is nowhere near the level of problem that old Poppy was. He's basicly an Amumu without a real gap closer, with an easier to interrupt ult, and less damage, but who's noticiably tankier and offers some extra utility to his team with the speed buff and being able to put the Defensive steroid on someone else.