Hello Phillip
No, it is not a myth if your design is using a mature process, a mature design library, all design parameters are well within spec and the design is made for manufacturing and test
Yes, it is a myth if your design is using a new process, new library, stress all design parameters and forget that it shall be manufacturable and test able.
If you want to test a design house ask for the number of designs done with the process, library and the design team. Get some references and talk to them about the pro & cons. If you justify the risk high, make a business model where the design team share the risk with you.
Henrik