Here are the 3 main differences between US-made tubs and real japanese tubs which make the two products difficult to compare.
1a) almost all US made tubs are coated with urethane. The look is natural but may feel a little cold and hard (like plastic) and have no aroma.
1b) Japanese made tubs are un-coated. You can feel the warmth, aroma and soft touch of wood but if the environment is too humid they may develop mold stains.
2a) US made tubs are assembled as a cabinet (see the corner mitered joint) and often made of finger joint glue-laminated wood.
2b) Japanese tubs are and are built like a boat and are made of solid wide lumber laminated with dowels
3a) US made tubs are made in red cedar, hard woods (like teak) or port-orford cedar (most of the makers call it hinoki – but it is not.) Please note that hinoki usable as construction material grows in Japan and Taiwan only.
3b) Japanese tubs are made with japanese softwoods (hinoki, asnaro, sawara, koyamaki). For flooring or wainscoting we use man planted lumber while for making tubs we use exclusively 250-300 year old trees lumber from primary forests.
Well seasoned wood is very stable but in extreme conditions of dryness it may warp or crack.