ECN isn't a solution so much as it is an optional side-effect of routers deploying AQM—the real solution. Widespread deployment of AQM depends on having an algorithm that is both highly effective and requires no tuning or configuration. ECN deployment further requires being confident that broken routers won't cause problems and making a decision of where to put the threshold for marking relative to the threshold for dropping. You want there to be an incentive for endpoints to negotiate ECN, but you don't want there to be too much incentive for endpoints to declare ECN capability and then not respond to marking.