Yep everything you said is true about side loading and scrubbing however they use them on commercial semi-trailers with the same issues and more weight. The height required was 2" not 6-8" which changes drastically the leverage and amount of strain on the U bolts. The amount of side scrub on a tandem is a lot lower than the same pressure generated on a tri-dom suspension using the same hardware. If the owner wishes to pay for a sub structure it is the superior way to do it however it does add substantially more weight. If I was going to spend the money on a substructure for $1500 dollars more than his quote you can have real trailer air ride.
