Plot of a larger scale means the inclusion of more parties, if a party or a group is then excluded the story should communicate clearly why that is. Which this game does not.
Wrong. When the big boys pull out their big guns, the little guy cannot compete. How in heaven's name is the Scoia'tael supposed to keep up with literally whole continents waging war against each other? Infantery, cavalry, artillery against a couple of elven archers sitting in trees. C'mon now, their best choice of action is to retreat.
The Witcher series is a game for adults. If an adult cannot figure out that a little guerilla resistance force - hardly organized or equipped - cannot compete with literally two big ass countries, then I am sorry, but this one's on the player. The Scoia'tael were always dependent on something or someone. In TW1 they were dependent on Foltest granting their demands, just to have them stripped the second he changed his mind. In TW2 they needed Geralt, Saskia and a whole state to be of any relevance. In TW3 there weren't needed for any big party involved, so their relevance vanished.
I mean, the game does communicate this on several occasions. There is a quest, where you find out that the Scoia'tael are attacking human caravans so they can survive in the woods --> How the hell do you expect them to play any major role in the bigger picture? They cannot even survive out there on their own.
You see several squirrel camps scattered across the map, literally implying that they are there but hold no power whatsoever.
Even the Redanian soldier/witch hunter acts surprised when you tell him the Scoia'tael are behind the attacks on the caravans, because he thought they have been eradicated already. Ask Roche, he'll tell you the insignificance of the Scoia'tael in the current situation.
I do not know how many more hints one needs to conclude the Scoia'tael are nowhere near to being relevant for the game.
If that is what happened then it is one example of the things the game should have communicated to the player. Not leave all of it up for speculation and assumptions.
The game does not need to tell you everything. It require you to read between the lines, to think for yourself. Otherwise you are turning the Witcher saga into another average RPG for the masses.
Nobody told us who Ciri was before TW3 and one was able figure it out by simply playing the game. The Witcher series is good, because it did not spoon feed you everything. Why should it change with the third game just because some seem to have a hard time to use logic/ use strawmans to argue the validity of their opinion.
But for the sake of discussion, let's pretend the Scoia'tael are in some position of power. What should they do? Join forces with Radovid and defend the North? Yeaaaaah, no. If TW1 Scoia'tael-Ending taught us anything, then it would be that monarch's cannot be trusted. Especially not the Mad King, who did not stop killing another monarch simply because he wanted total control. They'd be used as tools and disposed as such.
Should they join Nilfgaard and help them conquer the North in return for some kind of deal? They've been used as pawns once by Nilfgaard already, so I really doubt they'd go for that a second time.
Should they act as an independent, third force? That'd be stupid beyond any reason from a tactical standpoint and not fitting with the idea of choosing between two evils in The Witcher. Iorveth and Saskia simply have no place in this. Their place was in the Pontar Valley and that one got run over by Nilfgaard already.