I could be botching the terminology but the ship has two different types of propulsion systems, one similar to a jetski and a second one that I guess is more like a bow thruster  but big enough to propel  a ship run off diesel/gas turbine engines. The ship has a sorta of transmission system that and run the two propulsion systems together for extra speed but similar to a Ford lol the transmission system is doomed off the lot to imploded the old saying can't find them grind them and build your own.

Anyways these ships would be under way and they would engage the transmission and boom both propulsion systems down to limp mode and the ships would need to be towed in to port and taken out of the water or would limp back and spend months trying to solve the problem.

They put rules in place that the ships that werent broke couldn't engage the systems under certain circumstances. But ya these ships would become slower then the whole fleet when they were meant to be faster then most of the fleet.

As for the modular design it was like the IKEA of ships the idea was a two month turn around to swap modules the first swap took nearly a year the second 10 months.. there were countless problems they scrapped the idea all together.

As for the Zumwalt I know they have a third going into sea trials, and because the ungodly amounts of money they spent the navy is hesitant to really discuss the amount of short falls it's having they are desperately waiting for that rail gun system to be fully worked out and ready for use.

My opinion is the navy will eventually work it's way back towards surface combat ships because every country has something equally as powerful enough to take out a ship what we don't have any more is the brute force of naval gunnery to get up close in personal should we have to.