As far as Project: Eternity goes, I agree - there are degrees.
That said.
In a a general sense, I disagree. It is possible to take the situation you propose, and tell an engaging story with it (albeit probably not the kind of story appropriate to Project: Eternity.)
If you're familiar with a Pen & Paper RPG called Exalted, you might recognize where I'm going with this. Essentially, a fight doesn't have to be challenging to be part of an interesting story - if the rest of the story is on board with the assumption that "this fight will not be challenging". It helps to put a lot more focus on the 'grand scheme of things' concerns that we would traditionally consider the aftermath.
So, let's say you have your story of three hundred goblins reaving through the countryside. The party rolls up, wipes out the army with a brief bout of cataclysmic spellfire. The army is blasted to shreds, its few survivors routed and fleeing. The townships fall over themselves to celebrate your name. Local politicians try to contract the party to leverage similar power on other problems - problems which are maybe not quite so cut-and-dry, morally speaking. Maybe a particularly black-hearted aristocrat tries to strong-arm the party into working for him - people like that can always find ways to bend those who think themselves mighty over a barrel. A loved one held hostage, a slow-acting poison in the wine-glass to which only they hold the antidote, simple lies and half-truths...
Alternatively/additionally, a week later, goblin refugees start streaming into town. Women and children, starving and sick. That 'army' that was reaving the countryside? That was every able-bodied goblin, out in the world, trying to plunder enough food for their family to eat this month, because their crops have failed (possibly ruined by Something Worse).
The townships are reluctant to take in the families of the ugly, stunted creatures who not a week past were cutting a swathe through the countryside. Still, they're mostly good people - they could be convinced. If somebody tried to convince them. Somebody who, say, feels responsible that hundreds of sentients are starving, filthy and driven to begging for succour from their enemies.
But this creates further problems, because the townships (still recovering from the goblin raids, mind you) don't have enough food to tend to all these people. Hm. How to solve that?
In summary: "A challenge of non-trivial difficulty" is one route to an engaging story, but it is not the only one.
I'm not suggesting Project: Eternity take a leaf out of Exalted's book in this, mind you. I just find argument intellectually stimulating
Agreed.