It works with my R5 II, 600 EX RT II and EL-5. I mostly use flash outdoors like in the jungles so I don't know about RF interference in congested areas. Is that your concern? It was a common complaint a few years ago and then there was a FW update to the ST-E10 and some of the newer flashes to improve communication. I don't recall that the older flashes had upgradeable FW or if the ST-E10 FW change was enough.