Well, one issue, the cost. It costs three Nightshade to make one, and it costs one Nightshade to make a Spectral Flask of Stamina. With the mats needed to make 30 stamina flasks, I can make 10 power flasks.
This is way too little, especially for raiding. While it does objectively increase your personal power high-end content is balanced around you having it until you overgear the content. Lacking it in those circumstances is never an increase in your relative power, lacking it is a decrease in it instead.
Required X should never be expensive, it needs to be cheap to ensure that people have it always, else they reduce their own relative power, even if some things are rare.
