Plenary Indulgence is a very uncommonly used spell, and there's so many other things wrong with it, and maybe that's why I've never seen this brought up before, but it has a huge delay with its heal.
It's actually a bit longer than the Benediction heal delay that they've never fixed yet. It doesn't act like a insta heal at all, so on the offchance that I somehow did have 3 stacks, and it somehow didn't fall off by the time a tankbuster was coming, I can't completely rely on it because of its delay.

I know this is the last thing people worry about with Plenary Indulgence, but it's really something that shouldn't go ignored. We now have two "insta cast" spells on WHM that aren't actually instant casted because of animation delay, and that just doesn't look too good.