I was closely watching all matches on WC2014, and you are right, most players were not "struggling".
But there were excessive ammount of situations when players were like surprised with rebound of heavy-spinned balls. Completely missed topspins, crucial serve errors and receive errors.
In my personal opinion, there is large effect of new poly ball on players.
Effect is more noticeable in situation when actions of players are based on automatics (serves) and reflexes(powerfull topspins, smashes etc).
Also, I am afraid that this effect can be amplified by differences between balls of different batches and different vendors.
And for sure, there is psychological effect of new ball. Players will be saying that problem is strange rebound and effect of ball but not their reflexes and technique.
So it will take a year or few years to alleviate this effect via improving quality of balls, training for new balls, and work out reflexes for new ball.