Lightburst, the replacement talent for rhapsody on the priest class tree is objectively a worse talent than rhapsody, and it’s addition is particularly a major nerf to holy priest in mythic plus
The new talent buffs holy nova damage by 400% and healing by 500% but makes it go from having no cooldown to having a 30 second cool down. The enhancement to holy nova is the same as 20 stacks of rhapsody but with a bit more healing, however the time to reach max stacks of rhapsody is 20 seconds, while lightburst is a 30 second cooldown.
This is largely inconsequential to discipline and shadow priest, as if they were to talent into rhapsody it is only to press holy nova every 20 seconds when at max stacks for a moderate aoe heal
Holy priest however has a very specific rotation during apotheosis in mythic plus that allows them to output large amounts of dps, and is core to the spec in this content. with 2 talent points in light of the naaru and 1 in holy celerity, during apotheosis after using holy word chastise 2 instant cast holy fires from empyreal blaze followed up by a holy nova resets the cooldown of chastise and allows you to repeat the rotation. This is how to optimally do damage as a holy priest.
Removing rhapsody does not make this rotation impossible but it makes it objectively worse. Either the player does not talent into Lightburst and the gameplay is identical to how it is currently but each holy nova does not have 1 - 3 stacks of rhapsody and basically receives a blanket 40% nerf to holy nova damage. Or the player does take lightburst and only uses holy nova once during the apotheosis burn window and instead has to use smite for the remaining duration. Which causes this rotation to do significantly less in multitarget situations as you would be pressing single target filler instead of multitarget filler. And even on single target the cast time on smite reduces the total amount of chastises and holy fires that can be performed during your damaging cooldowns.
The cooldown effect on Lightburst also impacts holy priest in raiding content focused on healing, as currently holy nova having no cooldown is essential to its function as a non targeted and spammable movement filler for damage and healing
Rhapsody in its current state fulfills the exact same function as lightburst but allows the player flexibility in how they want to use it.
The tuning on lightburst should be buffed to be in line with the cooldown of maximum rhapsody stacks. If the cooldown is 30 seconds then the enhancement should be increased. Additionally this should really be a choice node with either rhapsody or a passive talent that buffs the baseline damage and healing of holy nova by a smaller amount, but allowing the player to keep it as a no cooldown ability