I think I have tested both these brands some time ago and both were trash (have a long PCIe adapter thread where I vomit out my personal thoughts about the “quality” in the industry).
The absolutely only (!) passive PCIe-to-U.2 parts that really are PCIe Gen4-capable (meaning absolutely no PCIe Bus Errors even after 24 h of full transfer load) are newer models from Delock:
- Delock 90081 (1 x U.2)
- Delock 90091 (2 x U.2)
- Delock 90092 (4 x U.2)
That doesn’t mean other quality models don’t exist, I’ve just given up on looking for them. You don’t need a “test of time” to check how good adapters are regarding PCIe Gen4, just a motherboard with enabled PCIe Advanced Error Reporting option in the BIOS, on AM4 systems this option was only functional for PCIe lanes coming directly from the CPU, not from an X570 chipset, for example. As mentioned I’m still ignorant regarding AM5 platforms, the earliest I’ll check it out is Zen 5 (or whenever they upgrade the CPU-to-Chipset interface to PCIe Gen5 x4).
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First off: If you’re only using a single PCIe x4 AIC adapter for a single U.2 SSD you don’t need any PCIe Bifurcation support from the BIOS. That comes into play if you want to operate multiple x4 SSDs in a single slot (for example 2 in a x8 or 3 to 4 in a x16 slot).
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The option to manually specify the PCIe Gen of a given slot is unfortunately optionial for manufacturers, most of ASUS motherboards have these options, but for example an ASRock Rack X470D4U does not.
Maybe you could ask Supermicro for a custom BIOS that shows these options. They 100 % have such versions for development/testing purposes.