One of the larger issues is it is difficult to repair something made with ancient techniques in a way that does not make it blatantly obvious it has been modified.

For example it was only recently discovered that what was thought to be a case of poor quality control in Roman mortar was actually intentional and allowed structures to effectively "heal" when their concrete cracked.

As a result, you almost have to go back to ancient techniques to make a repair or improvement that looks the same, and its as simple as the fact that a hand-sawn board looks considerably different than a machine sawed piece of wood...which means in the end...its going to be really expensive.