The 6 second cooldown of soul reaper is obstructing for all 3 dk specs rotations, especially frost and blood who lose valuable resource generation when using it. The gameplay benefits of “hold your gcd every 6 seconds no matter what” are minimal.
The 5 second delay also adds little to the spell other than the opportunity to estimate group dps to ensure the first soul triggers at 34.9% so that with perfect play you can fit in another soul reaper before the target dies. This can feel random and unrewarding when the first soul reaper triggers >35.1%, or when the target dies with <1 sec of soul reaper left to tick.
The 5 second delay also ensures that the ability will never be usable in pvp. Healthbars in pvp change drastically within seconds, guaranteeing a target will be >35% is quite literally impossible and it wastes resources if cannot be guaranteed.
Soul reaper could be changed to be only usable below 35% but drastically increase the cooldown and damage so it feels like an execute and not another plate to spin. This way the ability feels rewarding to press, and doesnt obstruct the rotation of all three specs.