They can be but not for the reason most people would think. Here's my experience with mine.
Kawasaki coils use carbon track plug wires, simply a string that is carbon dust impregnated in the sheath. My engine would run rough at times and then one day one cylinder just stopped firing. Coil tested good but there was zero connectivity thru the cable. Removed the plug rubber cap and saw that they used a crimped over connector with prongs that penetrated the sheath to make contact with the string. Except when I looked closely there was no string visible at the end of the sheath. Cut it back past the crimped prong points and there was the carbon string. Now I could measure connectivity. Got a pair of these and everything started working again. These have a wood screw type thread that runs up into the sheath center core and makes much better contact than the Kaw setup. Not saying this is your problem but just something to check and a solution if it is.
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