It's broken under present tuning and likely to be either irrelevant or degenerative under almost any other tuning point, which would make it at least wasteful, even if not necessarily stupid.

Let's assume for a moment that a healer can put out ~25% tank HP per GCD via Cure II or equivalent. This means that any extra free HPS via healer CDs just can be converted into damage so long as the raid has a DRK. Thereafter, moreover, it means that each healer can sacrifice a GCD of offense for a GCD (effectively 351 to 450p ST or 182 to 195p AoE due to Maim and Mend) with an extra Shadowbringer (600p ST + 450p AoE), for the cost of just 600 mp per GCD. That means the healer pair of, say, AST-SCH, can easily squeeze out at least some extra 2600 raid potency per minute unless Shadowbringer were nerfed even assuming there wasn't a single bit of oGCD healing to spare before (else, higher still). And if nerfed, then where would be the oomph of DRK's level 90 ability?

And if you're adding an extra 6 Shadowbringers per minute via Delirium for free before even having any HP cost... it'd just be a spammy noodle of a skill even before balancing out the healer-based stuff.