Why would you choose a system with a single Gen 2 PCIe lane xor bandwidth-constrained USB 3.0 as the only high-bandwidth I/O to build a storage server?

I ask as an owner and regular user of several Pi models, including a 4.

With that said, assuming you choose PCIe, acquire a decent HBA, and build it into a decent enclosure, I'm sure it'd work as well as typical low-end off-the-shelf NAS boxes, and wouldn't cost that much more in time and materials to set up.

It's possible but you're limiting yourself to 2 or 4 UDB drive mirrors. If you just want a zfs storage pool and aren't too picky about anything else, it works, but if your list of expectations goes beyond a place to dump data, you're setting yourself up to be frustrated later.

My home net has several pi3 and pi4 systems that boot off a small SD card and then mounts working storage via iSCSI, which works great for how I use them. There's no reason I couldn't serve that off a pi, but their limited IO and memory makes them a poor choice.