I'm not too concerned about acetal in the design, box ways provide damping so they are very well suited to an aluminum machine. You also have to remember that a ball-screw has only point contact whereas an acetal nut has area contact, so the loss of rigidity might not be as bad as it seems at first glance. Either way, I wouldn't plan on heavy cutting with a 5 axis machine.
Feature creep is dangerous, I generally prefer it when a Kickstarter design gets simpler rather than more complicated.
How many people are on the SwissMak team?