Originally Posted by FBinNY

Long spiral (low helix) or so-called compressionless housing was developed for it's superior ability to transmit position rather than tension. It as long considered unsuitable for brakes for two reasons

1- at high compression load the ends will try to extrude through the conical bottom of brake fittings and ferrules. This is correctable with a proper ferule with a reverse conical bottom which forces the strands toward the wall rather than the hole.

2- and the more important reason, is the mode of failure. A high helix, aka spring coil has the coils sit on each other constrained by the wire passing through the middle. In the event of failure a single coil may (very rarely) over another, but in general the cable depends only on the steel for it's compression strength.

Compare with the long spiral where the strands run lengthwise. At high compression the strands want to buckle outward and are constrained only by the plastic cover. If the cover is cut, UV compromised, or otherwise damaged the cable will buckle under load bursting and failing completely. Given how brakes work, one can rest assured that this kind of catastrophic failure will happen at the worst possible moment, when brakes are most needed.

This is why long spiral housing was often marked "not for brakes"

BUT

Knowing the market preference for long spiral housing, some makers started winding a ballistic, anti-buckle layer around the steel core (under the cover) to prevent failure. They're also gambling that other factors, liner wear, friction, cosmetics or simply boredom with the bike will have the owner replace the housing before the structure is compromised.

So long spiral housing is OK for brakes, but only if it's made for brakes, and used with non-conical ferules. Use of long spiral index housing made for gear cables is still unsafe.