I mean, the latter is not really an example of the former. If everyone is being really shit about taking damage and stacking grievous when they don't need to, going OOM is entirely possible between pridefuls. But even some pulls that have high aoe damage, even if people
are pretty good about avoiding stuff, it isn't abnormal for people to end up with a lot of grievous and it can be hard to catch up or sustain mana, especially for some classes. It's hard to make any kind of blanket statements about how hard/easy grievous is because it is so super dependent on the healer's class and corresponding toolkit, and also how good the group is.
I understand well how grievous work, but it's one of the few weeks where mana is sometimes an issue for me (bursting being the other since it fucks leaving combat for sipping in addition to damage). I might end a Prideful spawn running on fumes, so it's easy to see how if things had gone slightly worse (one missing interrupt, one DPS clipping sanguine and pushing their stack up, etc) might have resulted in deaths that just snowball. And I'm a resto shaman -- one of the best equipped classes for grievous. I feel like a Disc priest would just need to sit this week out, in contrast.
That said, there certainly are a lot of people who don't understand grievous and/or can't handle it, not just healers (but also lots of healers). I DPSed a +13 run earlier today where one of the melee that got hit by everything kept demanding to be "dispelled"
(of course while simultaneously reapplying stacks as the healer did their best to get him to 90); by a 13 you should fucking understand how that shit works. I also have seen a lot of healers struggling this week, and already had two healers just DC or AFK in the middle of a run after dying
*, causing the groups to fall apart. Most weeks I might see one leaver max, but this is one of those weeks where they are definitely more common.
* WHY DOES NO ONE ACTUALLY HAVE THE BALLS TO JUST LEAVE. THIS MAKES ME SO FRUSTRATED BECAUSE THE GROUP ALWAYS WANTS TO WAIT AROUND IN CASE THEY COME BACK AND THEY NEVER FUCKING DO. ahem.