SDS is usually witnessed in small NTT, 1.5-3" when they are young and week. I dont think their is a huge problem with SDS and NTT's at 6"+. I mean their is bound to be some cases, as in any species of fish.
I THINK the main reasons we dont see many large NTT's are because first off they are the least desired Dat, so less people have them to begin with, and they are usually the first to go when someone runs out of tank space but finds a nice ST, or IT they would like to have.
2nd the "unexperienced hobbiest" who has interest in dats will go after thin bars because they are the cheapest. (thats how I started)
3rd - they max out at only around 12" and for any fish to hit their "max size" is not the norm.