The possibility to reward a piece of the Unity set for a WQ is determined at the spawn moment, it is hidden, guaranteed (provided that Unity is active at the moment of completion), and in no way depends on your personal luck or any hidden conditions (like Unity accessories, weather conditions, group questing, frequency of WQ grinding, etc.).The type of WorldQuest can be anything: normal, pet battle, anima conductor, pvp, professional etc.Rewards are not bound to a specific server, and are most likely world or regionwide.The slot is not specific, i.e. if someone got gloves for example, you can get gloves too, but if you already have it, you will get another piece of the set from the ones you don't already have.
Me with my wife play at 4 accounts. So we have 3 Maldraxxian characters.
When the time came to do set for Unity we frustrated for 2 months (because only 1 was ready for it). Low chance, you know. But when two another come to this point we saw something weird. All of them got a piece. We thought that a World boss have a 100% chance for a Unity-reward. But 4 weeks after this we repeated and got nothing.
Then Millung posted:
"Just created this account to confirm that Unity armour pieces can drop from World quests that are unlocked from the anima conductor. I specifically got the chest piece from the; Kill 10 slimes and 1 big one, whatever it's called."
We did this wq and all of 3 chars got 3 pieces.
Then next day Scarty wrote:
"I got a "Duty-Bound Wraith's Boots" from pvp WQ in Maldraxxus.(It's my first item from Unity after approx. 1,5 months farming)"
We got next pieces again and the conclusion became quite obvious