It occurred to me that "Shidt + T" even couldn't really unload units from ships at shore - that would leave cargo on its own on non-native (ocean) terrain. "Shift + T" to really unloading units, as opposed to current behavior of simply activating them so that user can unload them, it would need to move units to (random?) adjacent land tile.

Would it be better or worse for the usability if "Shift + T" behavior depended on whether cargo can exist in current tile or not - all units would get activated, but those that can exist in transporter tile would also get automatically unloaded to that tile. That would fix both problem of not being able to unload zero move_rate units, and the case where cargo should stay at the city it's unloaded in?
Or should this work (as I suppose it currently somehow does) that you first use "Shift + T" to activate cargo, and then "U)nload" them, or unload them using citydlg present units area.

I added nuclear bomb unit with move rate 0 - should not move itself but be transported by bomber - to my ruleset.

I seem to be unable to unload bomb from bomber. As I hit "shift+T", bomb gets activated but with "L" icon displayed and when I then try to move bomber away before detonating the bomb, the bomb moves with it.

Actually it's quite possible that unloading as such does not work at all - I'm not sure if I actually did all what I think I did, but a couple of days back I meant to unload defenders to the city the ferry had just arrived - as opposed to them moving out of that city after unloading - and later I found them from the ferry that had again left the city.