RE: adjustable (turnbuckle) control rods
The advantage of the turnbuckle is that you can adjust its length without removing it from the servo arm or control horn. Easier to fine tune. Then when you get the desired length you tighten a nut on one end (usually the std thread end). If you use a rod with the same direction thread you have to remove one end at least for mechanical adjustment.
Also, if you want ultra precision your accuracy on a rod with the same thread on each end is only that you can practicaly adjust in 180 degree increments of your clevice. The rod with opposite threaded ends allows near infinite adjustment.