I object to the idea that it would involve a smaller cast and fewer locations than GoT. Maybe then the last few books/seasons, but The Darkness That Comes Before at least starts off quite a bit more widespread than Game of Thrones. The thing is GRRM's narrative flows and balloons outward, whereas Bakker's (mostly) consolidates around a few key points after the first book. I doubt we'll see any big battles at all, honestly. That is incredibly expensive and even GoT couldn't really justify having more than one or two after it's first season. Make no mistake, Prince of Nothing is not A Song of Ice and Fire. It's significantly more impenetrable, downright offputting to most audiences, and its main claim to fame in terms of the general geek public is being part of a particularly nasty social media spat.
No, it really is smaller than the early books of GoT. I'm not saying the Prince of Nothing is small, but GoT was deliberately written to be as wide ranging as possible and the Prince of Nothing is more focused. Check the shear size of the character lists in the back of the books. The TV series condenses as number of characters or omits them to try and deal with that while the Prince of Nothing really focuses down to the area around the army and meetings in rooms after the first book in contrast to the Wall, Kingstown, Team Khalesi, Team Stark, and assorted other places where Brienne and so forth. Big battles are expensive, but the central cast is much smaller and the locations are fewer as far as shooting it is a concern.
We are talking about the *FIRST* book, where GOT is relatively compact. Why are you bringing up Brienne? GOT (book 1 and season 1) starts off with all characters in TWO locations. That's it. It gradually expands from there, but it is a slow expansion. "The Darkness that comes before" does not do that... It does the opposite. Characters travel vast distances to finally meet at the end of the book.
That is a problematic structure for an adaption, no matter how you slice it. After that things do indeed become easier, sure, but you've got to hook people early... People will tune out if they are not hooked by episode 2 or 3. GRRM wrote GOT with with a very cliffhanger heavy structure which made it easier to include some "hooks" at every episode to make sure people came back. Bakker does it as well, but not as consistently as GRRM, so there will be much more adaption work required to create a structure that has that "hook" after the pilot - and every episode after that.
It's not impossible, sure... On the positive side, PON has a trinity of truly excellent characters to hang a drama around - Achamian, Kellhus, and Cnaiür. Get the casting of those three characters right, and things could flow well from there. Fail in that casting... And you would have a disaster.