Aggregate waterfall via Synthetics shows you similar data to what you may get in RUM, but of course generated from Synthetic monitors. There are a few slight differences from RUM, however.
Since "WCD" is collected for every synthetic session, you will see data from all successful session.
In addition to this, you will be able to utilize function tracing in aggregate waterfalls, which will allow you to see how long it takes Javascript to execute, not just download.