It's all about friction with a cable.
Maybe they have improved over the years and are made with better friction reducing materials, but will still be prone to sudden catastrophic failure, in other words they snap or a nipple pulls out!
Some interesting observations here;
http://www.ukmonster.co.uk/monster/s...spring+balance particularly on post #15 comparing hydraulic and cable.
The comparison doesn't take mechanical advantage into consideration, but highlights how the friction increases dramatically with each deviation from a straight cable route. No such friction with hydraulic and the mechanical advantage can be manipulated with various piston sizes. With modern radial masters this can even be manipulated via adjustable lever fulcrums.
With a cable you're pretty much stuck with what you've got unless you can somehow alter lever lengths, and that's after all the cable drag!