They mention surge of light in conjunction with the legendary ring :P We can both agree that it's either binding heal or Piety for your main pick. I've played with the talent ring for a while as well (no tier shoulders or chest ment I was locked out of Archbishop's, so I experimented with various legendaries for a while), and I can sort-of agree with the free flashes being useful to buff up PoH in a pinch, but then, BH does the job even better than the ring because it's reliable, and it gives holy the one thing it really doesn't have right now; An ability that you can ABC without killing your mana pool. Here's a recent shivarra log (Note: I am in no way good at holy. I miss my POM cds, I don't heal for a ton; This is just to demonstrate how much you can actually spam BH without destroying your mana).
https://www.warcraftlogs.com/reports...casts&source=3
On coven specifically, it absolutely melts the debuffs - especially if one goes on you (as remember, BH is healing you + 1 random + target):
https://www.warcraftlogs.com/reports...24true%24false
While at the same time building up sanct/serenity stacks.
So while SOL is fun to have through the talent ring if you've got no other choices, prydaz/archbishop for survivability or velens for throughput are still a lot better. In fact if you do go for binding heal, the pants become very attractive as well, because with a longer cd on POM (and thus less POMs out), your renews are less likely to be overwritten so longer renews are "better". It's not a huge deal, but it's still worth noting and probably outshines any benefit SOL has, especially as the class ring lacks a stat budget.
(That said, I did find the constant free SOL procs from spamming binding heal a godsend in M+; Holy often lacks a way to react to heavy single target damage on a tank outside of serenity, because so much of the healing is tied up into a single burst holy word, and a 6 second HOT after the initial hit. Being able to spam BH while no damage was going out due to no mana cost, and "banking" two SOL procs for emergencies was nice. It's my usual go to for dungeons when my cloak is on CD, I have a gear switch to make sure I'm always benefitting from 2 legendaries).