Having the small driver boards makes it easier to get the board out to repair them rather than the huge board with dozens of connectors in the backbox. It also saves a bunch of wiring, unneeded complexity and manufacturing/labor cost.

As for the driver transistors, I don't like the idea of riveting them down or holding them down. If you look at the SPIKE boards Stern left the transistors standing. To me this will be the better way in the long run. There is little or no vibration under a pinball playfield except from the speakers and that vibration is present in the backbox too. There is shock under a playfield (shock is different than vibration). Shock comes from the coil plungers hitting their stops and in this case is not enough to affect something so light as a transistor that is supported in place by three heavy soldered leads which are basically holding it flush on the board. Something like a ceramic resistor that is not glued down would be a problem, but not a properly soldered-in transistor. Personally I would be more worried about heat expansion and contraction of the transistors and their leads with them riveted to the board. That will cause more stress on the solder joints over the years as they heat and cool and fight against the rivet and solder joints than the shock (or vibration) ever would. Also they will stay cooler with them mounted upright. The board is not a good heat sink and I don't see any metal there...

Anyway, I am splitting hairs here. Both the MMr system and the new SPIKE system are advances in pinball technology the will make them cheaper and easier to manufacture, more reliable, and easier to maintain and service.