It depends - soldered RAM (LPDDR) on iPads and soldered SSD on Intel Mac Minis can be repaired by replacing the chip. (But obviously it requires skills and specialised tools). So you don't necessarily need to replace the whole board. But if it is some kind of "integrated" non-standard RAM, like Apple uses on its ARM series SoCs, it is near unrepairable. So yeah, one should think many times before buying a computer with soldered RAM. (On the positive side, I've never had to replace any RAM in any computers I've owned so far - the failure point has always been HDDs for me).

in case of macbooks even if something like usb port controller failed is most likely leads to board replacement as well so is high change that even if RAM is fine it could be useless because some $2 component failure