Especially for siege, and especially if you have alot of them in the ship and you can’t drop them 1 by 1, transporting across a river is basically impossible.
They just won’t find a pickup/drop off point and it’s therefore utterly futile to try to use them.
Last game I got 8 bombards stuck in my transport ship and pixelaccurate tried half of the map on Confluence to get them somewhere back on my/my oponents landpart but it was utterly impossible to get them out of there…