The actual ram internally doesn't clock that fast, and many in fact run at the same rate as in the ddr4 days. The interface tackes the high rate down to a much lower rate but with more data flowing in parallel. But the ratio is locked, so if you overclock, the ram rate has to go up too, which is most likely what makes it unstable.
So the upshot is that if this 6400Mhz device is not overclocked, it shouldn't be unstable.
Max internal clocks on DDR4/5 are about 400MHz. There's a fundamental limit (rapidly diminishing return) to how fast you can drain and charge capacitors.
They try to make up for it by reading/sending entire row(s) at one time and sending quickly over the wire, but that's clearly reliant on cache line optimization of the program and doesn't do much to improve real-world latencies.
So the upshot is that if this 6400Mhz device is not overclocked, it shouldn't be unstable.