A lot of folks have discussed their benefits, so to dig into the compromises vs the EL SV/FP 12's, based on my experience with them thus far:
1) Eye placement. Out of the box, NL 12's are more difficult than other Swarovski's to just "walk into the image" without the occassional blackout or...
Just got the NL 12's.
High quality glass, no doubt.
However, I'm not convinced they are a big enough upgrade from the EL SV 12's. There are compromises that jump out at me. Time will tell.