For example, I've never care about dates before, but that turns out to be stupid, as I did not notice that most of the Italian units I've modified can't be bought in the stock Sandstorm campaign. During testing I've left the available/expiration dates blank and made most of the tests with the editor, but now that I've looked them up... the Semovente were prototype units from late '43-'44
Just really dumb of me. Anyways, now I've changed a number of dates with the Italian units to make them available to the player. But only for the last couple of scens (i.e. Kasserine, early '43), of course.
There's quite the number of units in OoB included that are for scenario builders and modders exclusively, it seems. None of them are used with stock content and can't be bought in the given WW2-timeframe neither. So I've unlocked some of them.
The problem is, that the "nopurchase" trait is listed within the "classes.txt" file (not units.csv) and - if removed - changes the unit's behaviour in the FOW for some reason. Try it. Maybe there's some work-around, but I don't know of it.