Yes indeed cables just transmit light. The question though is how much light goes in, how much light comes out and how much light does the strobe need to flash. For the EM-5 using the built-in flash and the YS-01 in manual mode you most likely will never have a problem. If however you are trying to get TTL to run correctly you might not be able to get it to work all the time and if you are using say a Nauticam trigger you might not be able to make it work at all. Home-made cables come in many flavors ranging from small fibers (1 mm) to big fibers (3 mm) to multi-core fibers (613, 217, 1000) and many construction techniques. I have an EM-1 Mark II and Nauticam LED trigger. It fires the Inon Strobes every time with a single 1 mm fiber but will not fire the YS-D1 with that fiber. It will fire the D1 with a coiled 613 core fiber every time though using the LED trigger. I have not taken the time to try to measure fiber transmission to characterize them but stories of non-multi-core fibers not working reliably are legion.
Bill