You lose efficiency in the powertrain right up to the propeller. Having more propellers (larger total area) is more efficient. A bigger factor is that by “blowing” the wing you can use a smaller wing, which reduces cruise drag - hopefully by enough to more than offset the loss in powertrain efficiency. Theory says it can work; practice is trying to catch up but hasn’t yet. Electric motors make it practical to try. Battery capacity can be quite small, so battery tech isn’t a big issue.