I worked on one of the industry teams that lost the competition for ACTUV Phase II, and I currently work with a lot of former nucs and submarine captains.
Given that the USN is currently constrained from practicing with active sonar because it might hurt the whales, there is no possibility we’re going to have an unmanned nuclear reactor or radioisotope source just sailing around. I can’t conceive of circumstances that would change that.
Not easy to communicate with things under salt water.
Depth is limited, speed and bandwidth limited.
Need to come up for orders (like, oooops, Iran bringing it home).