Solid round cord does not generally contain much, if any, grain side material which makes it relatively weak for applications requiring a reasonable amount of strength. I am sure there might be exceptions but I have not personally seen any. As such, I would be reluctant to use if for a lanyard. Braided round cord is another thing altogether. It can be very strong and is good for almost any application. High quality flat cord can also be very strong but be wary of the low cost stuff or material that looks dried out.
Randy