Priest have absolute garbage manna generation. Especially Holy as disc at least has rupture as bad as it has been nerfed down. Meanwhile monks have manna tea providing a near endless supply of manna in comparison. Priest heals are also very manna intensive. Especially flash heal which is a a must use to reset cooldown on Holy word serenity.
I do not raid so I can not say for certain how well priest manna holds up in that regard but in both M+ and PVP content, Especially as Holy, You are left dead in the water all the time with no way to generate manna unless you can get out of combat. It is so bad I sidelined my priest all together and only play my monk now as I never have to worry about manna. It makes me sad. Priest have always been lackluster at manna generation but this current meta has taken the cake. It’s just bad.
Perhaps if Shadowfiend was a 1 min cooldown it might be more competitive but even then, It would still underperform against manna tea. It wouldn’t even be close. especially since shadowfiend require you to baby sit it to make sure it stays on a target or it is just dead weight.