I used a rockpro64[0] with an off the shelf PCIe SATA card for my home NAS (4x4TB disks). So far I'm extremely happy with it (using it for almost 8 months). The rockpro is running debian/ZFS.
Interesting! I've been thinking of building a NAS and hadn't considered the rockpro64. Have you found that 4GB RAM has been sufficient for ZFS? I'm currently running a small ZFS array with 12GB RAM and the box never seems to use anything close to that amount, but ZFS has a reputation as something of a RAM hog.
I'm not using de-duplication, so that might affect if you only have 4GB RAM. Right now the box has 2GB free, and I'm always running a few extra things in it. Overall, for my use case it works just fine. The SATA card is plenty fast as well. I had a similar setup with an rpi3 and a USB<>SATA to host 2x1TB disks and doing a scrub in that pool took over 4 hours. The scrub in my current pool (4x4TB) took 22 min.
Cool!
Does it require a custom kernel?
it has had mainline support for a while now, there are even now daily official debian builds available as well: https://d-i.debian.org/daily-images/arm64/daily/netboot/SD-c...
That said, I'm surprised that they explicitly support installing Linux -- the post links a QNAP wiki page that explicitly uses the phrase "supported hardware" when describing which machines their guide to installing Debian works for. The case itself is pretty high quality for the purpose (no surprise), but I don't think I'd buy one just to wipe the custom OS and install my own on it. If it were me, I'd probably hop on over to r/DataHoarder, search there, and maybe make a "recommend me a case" post if I didn't find anything interesting.
I'd really love to do a NAS project with a Raspberry Pi, but the lack of SATA support really does it in as a base for a NAS system, IMO. If you're just going to connect everything over USB anyway, what's even the point of the Pi? Just buy one of those dumb external enclosures that holds 4+ drives.