In a hotfix late last night, Retribution Aura was hotfixed and now procs more reliably making it a real raid buff on all encounters!
In a hotfix around 6:30 PM PST last night, this was changed so that Retribution Aura can now proc off of spells as well, making it much more consistent.
Previously, Retribution Aura would could really only proc off of Melee attacks that bring the target below the health threshold, making it hard to proc reliably especially on bosses like Nymue which don't actually Melee other than the Cycle Wardens.
Compare the following examples of Mythic Nymue:
So having Retribution Aura now seems to be around at 1.6% damage buff since it procs way more reliably now.
Retribution Aura seems to work out to an average of about 2.3% while the effect is active according to Reloe, and the overall buff is about a 70% uptime.
Read more on wowhead.com
The effect is most potent at the start of the aura for the first 10 seconds.