This isn’t related to me having a question, but rather me confirming something that I know a lot of people didn’t know. 
2 years ago I did the Legion Balance of Power questline to get my artifact skin, and my entire guild + best friends told me that the “raid quests” where you had to collect shards from bosses, were only doable ONCE.
Meaning you could only do 1 run a week on either normal, heroic, or mythic difficulty. (sad face)
But then 2 years ago I accidentically found out that this isn’t true… And now today, people STILL keep telling me that I am remembering wrong. 
So here is my findings from doing this questline this week again on multiple characters!
The Emerald Nightmare questline:
This questline can only be done ONCE a week on normal or higher. Higher difficulty does NOT increase the drop rate.
The Nighthold questline:
The Nightshards can be farmed ONCE a week with a high drop rate on normal or higher. Higher difficulty does NOT increase the drop rate on this first run.
HOWEVER! You can do the last two difficulties for a VERY SMALL drop chance on each boss. Guldan however does seem to have dropped 1 Nightshard on mythic guaranteed for me in those alternate runs that week.
So with that CONFIRMED knowledge on my end, good luck on your Balance of Power questline.