I had a Yuba. Magnificent machine. 2 x 14 stone people on the back no flex at all and completely practical on the flat.
But I found the 67lb weight, including plywood deck and foot boards and mudguards pretty daunting and I let it go and got an electric assist bike which, because the assist drives through the gears, is excellent for load carrying, though not capable of big Dummy / Yuba loads without a trailer.
If the Clevercycles Stokemonkey electric assist had been available for the Yuba at the time I’d probably have gone that route. Exchange rate has of course pushed up prices a lot now.
A Yuba + decent hub motor kit would be about same price as a built Big Dummy and whilst not as ideal as a through the gears motor system it would be a very potent practical load carrier.