I don't know if that's the case with intel, but I've extensively tested my own 5900X on both a £250 X570 motherboard and on a £40 B450 motherboard(with the same ram) and there was literally no difference whatsoever. No benchmark has shown any difference. You lose things like PCIe 4.0 connectivity in that scenario, but the CPU performance was identical.

edit: just wanted to add - yes the VRM section on the £250 mobo is vastly superior so I imagine overclocking isn't even a contest at all. But in stock form the 5900X performed equally well on both.

That makes sense though, doesn’t it? Otherwise you’re not just benchmarking CPUs but also motherboards, which is another variable and makes drawing conclusions about performance messy.

It depends whether you want your benchmark to be fair or to be representative. I think there's a case to be made for both approaches.