Necklaces made with delicate yarns or cords should have metal findings or no closure. Tassels were used historically. But the finished cords should be softer and more flexible if they are going to be used to embellish the knitted or crochet garment. The yarn needs to be balanced when finished. Variegated colored yarn do not show off most knots well and are difficult to keep track of the threads while tying the knots.
Necessary strength is dependent on the intended use.
A cord may be weak if no tension will be on the final knots. Generally, elasticity is not desirable.