The durability is one of the biggest problems. The 1.12 server both I and the OP play on had to nerf shields to around 15 durability to make them balanced for PvP. Disabling with an axe lasts a very short time and its incredibly easy to track players with a shield as they only have to be visible to be blocked so they can't hit you from behind. Unless I'm in an Overpowered 1.11 fight I never go for an axe to disable a shield as its so easy to get counter combo'ed from it and die. Pretty much the only way to combat them is to hit and back off which, if done with an axe, uses up too much time before their shield recharges before getting off more than 1-2 hits.
In 1.10 that durabilty was more than doubled than what was in 1.9
Cube could always tweak with the Durability, that's not problem. Besides it's only 336 not that much.
Anyways players have to lower their shields if they want to attack right? That's gives them a disadvantage and also an opening.
Another thing is, Axes have a 25% base chance to disable shields for 5 seconds. If sprinting it turns to 100%. It doesn't rely on the cooldown, meaning that as long as you're sprinting and manage to hit the shield with an axe the user will get paralyzed. Giving you a opportunity to strike back using knockback or just dps.
The last thing is that you can simply do (in team games) is use your team mate as a decoy to strike the shield user from behind.
If Cubecraft puts it's Shield durability to 150 then Shields won't be a huge problem.