You can hit a RAM limit on some lower-end motherboards quite quickly depending on the memory controller and you might only get so far with GPUs as well depending on the type of PCIe slots.
I'm not sure what decade you have in mind, but for all the recent ones, the memory controller has both been on the CPU, and not been part of the differentiation between low-end and high-end CPUs for a given socket. So the only significant RAM limitation coming from the motherboard is if it's a small form factor board with only two slots instead of four.