Is it enough that Nico says they shouldn't be used as end fittings? And that they specifically say not to use them for sailboat rigging?
There are some major structual problems as well:
1) Nico's don't grab 1x19 well, so you either have to accept a significant loss in efficiency, or change to 6x or 7x which changes shroud
stretch negatively. So you need to go up in strength.
2) the only way to get full strength is to double them, but then you have a problem of getting equal preassure on each fitting. If it isn't exact you have created a hard failure point
3) 1x19 is reluctant to go around thimbles, and the right size thimbles are both expensive and pretty rediculous. You have to go to 40 times diameter to get to the wire breaking strength. So for 1/8 wire you would need a radius of 5 inches. Which won't fit into the turnbuckles.
4) crevice corrosion
with them has been a long time nightmare. They trap water
between the fitting and the wire and are almost impossible to get dry
Again they can be used, and on small boats you can probably get away with a designed in strength loss, but it is a major compromise to gain little.
The synthetic are great when done properly, a nightmare otherwise. The upside it doing it right isn't that hard, it just takes the willingness to do it right from the beginning.
First, you have to use Dynex Duc.
Second, it has to be sized to control creep, NOT breaking strength as with wire
Third, you must use properly sized fittings.
Assuming these three rules are done right, you basically get standing rigging that costs about the same the first time it is installed, but a fraction the cost for replacements
. Weighs about 1/5 that of stainless, and can be replaced in an afternoon of easy splicing when the line wears out.
Real life testing indicates a service
life of 8ish years for the line, with the aluminium fittings lasting pretty much forever.