The idea seems simple enough and the math to do these kinds of predictions is fairly straight forward unless there is some secret sauce under the hood that isn't obvious. I guess the value is slick UI / tool that was created around it.

One question that I do have about these types of tools... how does one evaluate how well it worked? I guess in the end as long as the timeline wasn't too far off then you call it a win.

I think a useful tool for project planning is not necessarily simulating possible time-lines (or a estimated window of time for the project to be completed) however to intelligently identify a set of critical tasks within the list which would have a high probability of totally blowing up your time line their deadlines are missed. In project management / scheduling there are often critical path items listed. However often these critical items are semi-arbitrarily picked by the PM and not backed by and deep insights.