Thread furled leaders .... that submerge ...
ne flyfissure -
Methinks your question is a bit broad to really answer, but I'll post a couple thoughts on the subject and expect others to chime it.
For background, I started furling my own thread leaders about five years ago. I have used them almost exclusively since then for dry fly fishing, nymph fishing, and wet fly fishing. The exception would be fishing tandems of large weighted stonefly nymphs under an indicator with mono leaders for a period of time until I furled a beefed up thread leader that would handle that load.
Regarding nymphing, I almost always use an indicator, but I have fished nymphs with the thread furled leader without an indicator on occasion. When using an indicator, it is placed on the furled leader, with the current speed and depth pretty much dictating where along the leader it is placed. Faster and deeper, the indicator is near the butt end of the leader. Slower and shallower, the indicator is near the tip end of the leader. The exceptions would be when using a dry fly as an attractor / indicator while trailing an emerger or nymph.
With a thread furled leader, which is very supple, drag has never been a problem. My casting, or mending, or other line control issues might have caused a problem, but not the leader itself. In fact, it is my firm opinion that the thread furled leader is the best way to avoid drag for almost any kind of fly fishing because it is so supple. Also, whether fishing dries or nymphs, a submerged furled leader does a better job of eliminating drag in most situations than a floating furled leader or one made from a stiffer material.
The thread furled leader above the indicator does a great job in achieving a drag free drift of the indicator, which means the nymph is likely being presented drag free also. The part of the leader below the indicator is also contributing to the drag free presentation by absorbing whatever currents there may be between the indicator and the nymph.
Another plus with a thread furled leader. For the type indicators I use, it is quite easy to adjust the depth from the indicator to the nymph, and if the right method is used to secure the indicator to the leader, it will stay in place.
John