As much as we all WISH they would… This is DEFINITELY not the case.
Even with using the same chip (which grants the ability to use all of the functions incorporated into the chip), there are and have been SO many variations on how and which functions of that chip is made use of. (Nuvoton’s NUC220 chip is an outstanding example of what’s described here)
On top of which, you complicate matters with firmware.
So, even IF they designed a great pcb, with well chosen supporting circuitry to support the electrical needs (power supply, regulation, monitoring, etc) they can “flub it all up” with the firmware. Granted, the latter is far easier to fix, as it (theoretically) requires no redesign of the rest. Simply fix the firmware error(s) and update.
But how many times have we seen “firmware upgradeable”, yet never had an update released?
Things to think on.