In the current form federal EU would be someting like having an unelected powerful executive branch, and a semi-elected weak legistlative branch. Furthermore the populace has very little idea about what is happening in the EU and who to hold accountable, partly because the media doesn't cover it, and partly because the processes are extremely convoluted and quite opaque.
Such "democratic centralism" bureaucracy probably would have benefits like more stability for long term strategy, swift execution of policies and coordinated action, but it's also very prone to corruption and elite capture.