It's a tradeoff. The old SP1s and SP2s with Wacom digitizers get funky at the edges because of how Wacom's technology works. IIRC it's a big array of sensors behind the screen, and they have to extend past the edges of the sensing area to keep accuracy workable across the surface. If you're out at the edge of the grid it can't measure the pen's position accurately.
With Wacom's Intuos tablets you've probably noticed they have a really big bezel. Maybe 2-3 inches around every side. It's not just a physical design decision, they need that in order to work. MS couldn't accommodate that on the surface, so the accuracy at the edges dropped off. And this being Windows with a lot of software not designed for touchscreens, that meant you were trying to poke at tiny toolbar buttons in the least accurate region of the digitizer.
Ntrig solves that, it's let them at least be consistent across the whole surface, even as bezels got smaller and smaller. But like you said, the peak accuracy of Wacom in the middle of the screen probably beats it.
I sold my SP3 to buy a 9.7" iPad Pro when those came out, mainly because I got annoyed with the digitizer. If you draw a line slowly you can see it get visibly pulled back and forth along the digitizer grid. To get smooth lines you have to do fast sweeping strokes, which I really didn't like.
For reference, lines drawn on a SP3 in OneNote: http://i.imgur.com/bxpnPSv.png
In an AMA on reddit the Surface team attributed some of the pen performance issues to "capacitive coupling" making the pen not detected as accurately if you let the tip touch a ruler, so those are drawn freehand. Even so, you can clearly see the lines get pulled into the pattern of the ntrig grid.