The "only" true advantage of a PCB-based design is its HUGE potential for automated production and testing, with all the obvious implications of this fact, such as a cheap final product and repeatabe parameters. Other factors (cooling, impedance of power planes, signal integrity, reliability) are inferior to what can be achieved using older or more exotic techniques. For most of the time it is not worth the effort, as the PCB is good enough, sometimes going full custom pays off. Printed Wiring Boards, for one example, they still seem to be available from Hitachi. A piece of good coax connecting various points on the same PCB is another example of a superior hybrid solution. With a planar trace you just can't get even close to the performance level offered by a dedicated solution. This is all clear.
But, at the other extreme, I occasionally see military-grade electronics mounted on an FR4 substrate, but without the use of *any* copper layers. Absolutely everything is manually wired. This is probably the best example of this wiring obsession:
but these are good too:
What were those guys trying to achieve, if even a single copper layer would have allowed them to route all the obvious stuff and apply wiring only where truly needed? These are avionics/military designs, so I presume they were made by real experts, not a bunch of loonies.
Best regards, Piotr