I have run both the 1106 and the 2106, sometimes at the same time and in my experience there is no substantive difference in the depth accuracy between the mechanical and electronic counters although I have not tried the mechanical counter on a 2106 modification. As I understand it, the riggers are calibrated for accuracy using 300 feet of standard Scotty steel cable. If you put on more or less cable, especially a lot more cable, say 500 feet plus, they will be out a bit. If you want to know exactly how far out in order to compensate, let the rigger ball down to a flat bottom at 100 feet on the sounder, in slack current and wind, while the boat is stationary and compared the counter to the sounder depth. This was useful for us to establish a baseline when we have run much more than 300 feet on the rigger.
If you are concerned about depth accuracy (actual depth of the ball in the water coloum) in an actual fishing situation you may find that blow back and drag are usually more critical than counter error and there are many factors that contribute to that.
If we are on a flat clean bottom and want to fish close to the bottom in a strong blowback situation, we sometimes forget the counter and drop the ball to the bottom till it hits, then bring it up a couple of feet and wait for blowback to push it up and then drop it again and bring it up a foot or however many feet from the bottom you want it to be and then compare with the counter and sounder.
We have not had a lot of problems with the electronic counters. Eventually we killed the battery on an early generation one and had one that failed early that may have been from a bad batch which the Scotty guys gave us a replacement for free at a boat show. Overall I am happy with them and I have not yet tried the newest generation electronic counter.