On Apple silicon some high-end pro cards may still be unsupported - like ProTools HDX cards I believe just got native support. They also don’t support PCIe 5 which could be added with the Mac Pro. I’m not totally aware of the full status of card support though.
Only cards that would benefit from PCIe5 would be (GP)GPUs, M.2 NVMe SSD-based RAID cards, and probably some high-end multi-port networking/fiber channel cards; the Blackmagic Design 8K video I/O card is PCIe3 x8 and the HDX audio cards are PCIe3 x4...?