How come Morellonomicon grievous wounds last 8 seconds?

Because it has the prerequisite of them being under ~40% HP to apply, while executioners has no prerequisite.

Because it only kicks in if the person is below 35% HP.

Contrast that to Mortal Reminder, which lasts 5 seconds and is applied any time you deal physical damage.

I think Grievous lasting more than 4 seconds on any item is ludicrous. 1 autoattack soaking up almost half of Mundo's ult is insane.

It also has a difficult window to actually apply it. Instead of on hit it has a %hp requirment before being applied. Most scenarios will mean that the 8 second duration is pointless when you are probably 2 seconds away from dying when it is applied, if even that long.

Plus it is probably still kinda balanced around spell vamp items being a thing.

It's an absolutely pain in the ass to use.