As long as the campaign is done so that you don't sink huge numbers of ships (in the PTO, anyway) it's not too bad. Unless you keep track and know that there were only X Mutsuki DDs you are unlikely to think you've sunk all of them.
The "zebra" units (rare and interesting compared to "horses"
) are another story. 2 Yamatos, zero possibility of there ever being more. If you sink one, you should never see 2 together, much less sink 2.
Makes doing a really compulsive campaign hard. It would also be nice if you could make a unit within a group disappear. Right now you make a Battle of Midway for the campaign, and the only way to sink the flattops is to put enough bombers around that they always all sink. Course you'll likely sink everything else, as well.
(actually, I just got an idea that might work... damn complex, though.)