For as long as I have been watching progress in tissue engineering, the primary and most important barrier to building organs to order has been the inability to construct vascular networks. A network of capillaries must exist for blood, and thus nutrients and oxygen necessary to cell survival, to reach more than a few millimeters into a tissue. In live tissues, hundreds of minuscule capillaries pass through every square millimeter, considered in cross-section. Replicating this level of capillary density in engineered tissue has yet to be accomplished, with even the more advanced technology demonstrations falling well short of this goal.
Well funded initiatives such as the effort to produce genetically engineered pigs with organs that can be decellularized for transplantation into humans, or the application of decellularization to donor human organs, should be considered as attempts to work around the vascular challenge. That is why they exist. If a suitable vascular network cannot be produced from scratch, then the existing vascular network in an existing organ is the only viable alternative. It remains to be seen as to how long these approaches will be needed, how long it will take the