Regarding the underworld, in our example yes it was more easily fought over, but look at
gameID=323, the ogres held a good lock over the area without needing to sacrifice too many overworld units. and as for your 10-4 Unit-SC argument, remember that person also holds the lock on a second half of the board, making it a strong stalemate position as I unfortunately had to find out.
Speaking of Stalemates, to elaborate more on the northern area, it's not just an easily defended line, it's multiple lines that can bend and merge to fit a shape that works for the number of units available. again looking at the other game, despite taking full advantage of the NMR, I was only able to push a little bit before it snapped back. I think what makes the line worse is how easy it is for one country on each of the two continents to dominate the region. two games so far show this effect.