Quoted from Durzel:On the flip side discrete boards mean that if your sound fails then you replace the audio board, at a reduced cost compared to replacing the power board, CPU board, etc.
On a single board system if your sound fails you have to replace the one board that does everything, even if everything else in the game is still working, at a significant cost.
SAM and whitestar are worse in this regard. They have a total of 2 boards with everything on them. One has the CPU, sound and display driver (some titles have a separate power supply and driver board for the display), the other has the power supply, solenoid drivers and the lamp and switch matrices.
Spike has a separate modern switching power supply (no transformer). One board with the CPU, display and sound drivers. But the real beauty is the distributed boards with switch inputs, controlled lamp outputs and solenoid outputs. This GREATLY reduces the complexity of wiring in a game (short runs from each board under the PF rather than many long runs into the backbox). Plus for those people that cannot replace a driver transistor themselves can remove a small board and either buy a new one or send off the small board for repair. Much easier than sending off a 10" by 14" (or so) SAM or Whitestar power/driver board.
Granted, Stern charges too much for replacement boards ($200+ for a node board but what manufacturer doesn't charge too much for replacement parts (see: automobiles).