This aircraft (which is hard to find an actual picture of) seems like a really inefficient design.
Each engine introduces substantially more weight and drag than does simply increasing the propeller diameter, so usually designers choose to minimize the number of nacelles and engines to what is required to accommodate an engine failure in flight.
This prototype seems to be taking the opposite approach. While it looks interesting, it also seems like it would have a lot of drag when it transitions into horizontal flight mode. This is probably what TFA means when they say "The biggest challenge yet to be overcome will be whether the full-scale model can meet the performance objectives DARPA initially set for the project."