@walking-dude I would say that for this kind of deck you might want to direct your attention to Red, since damage always stacks. And let's talk for a moment about damage shall we... because in my personal crusade against Null Rod, I've thought about this a lot.
In some sense, every card "stacks" since having another card in hand tends very strongly to be positive value. And having a second, insurance Null on board tends to insure the effect much more than just one. But for the purposes of this discussion, Null Rod does not stack. I have no problem with your definition. I've used it myself many times.
In the same way, Crucible does not stack... But now lets look at Ramunap Excavator. It's effect is the exact same as Crucible. Yet drawing a second Excavator is actually a lot better than drawing a second Crucible (in most cases) because Excavator does damage, and damage always stacks. Two Excavators do twice as much damage as 1, and 4 per turn, while not a huge clock, is 5 turns in the best of scenarios. It's at least real.
So now we have other cards like Pyrostatic Pillar. It stacks. But also it isn't a creature, so it can't attack. Is a 2nd Pillar on board better than Excavator number 2? I dunno. In that particular example it probably depends. If they have a ton of blockers on board than Excavator is probably just bad, but if they can just kill you through Pillar by landing up and hitting 1 huge spell... so it depends... But I would argue, all effects being equal, the Excavators should be better, since they are 3 effects, and the Pillars are 2... that is, the Excavators in total provide 2 damage + 2 damage + the land from graveyards effect.
Now this is all theory, and it's totally dependent on the quality of the effect. Oath of Druids doesn't really stack. (Ok it does, maybe this is a bad example, but you rarely try to or need to trigger both.) Do I mind drawing 2? Not really. That effect is nuts. Some creatures have no effect and are just super efficient damage... Delver of Secrets.
My point is just that there are basically 4 levels of "stackability", No stacking not a creature (Null Rod), No stacking creature (Excavator), Stacking but not a creature (Pyrostatic Pillar), and the ultimate... Stacking and a Creature (Eidolon of the Great Revel).
I just want to point that out. The fact that they are creatures, makes drawing a second one a live draw, if you plan to kill them with damage. Because more damage is always better, with the potential exception that they were dead already.
