Originally, all R/C boats, including hydros, did run hard shafts. When I got into scale boats in the early 90s, flex cables were just starting to show up. What made the flex cables desirable was that the cable was easier to install and set up. Hard shafts required nearly perfect alignment between the strut, stuffing box and engine coupling. Any variance in alignment resulted in power robbing drag that could and did affect the performance of the boat. Since cables were able to bend, alignment became a non-issue and actually allowed the thrust angle and lift to be optimized without having to totally remove the stuffing box to make a change