Software RAID = simple. At least when I built this machine nearly 10 years ago. Since then it's been rock solid, if not blazing fast. There were also horror stories of hardware raids failing due to controllers failing; or subtle differences between different revisions of RAID controllers. That said I also had budgetary restrictiions at the time that made software raid attractive.

I looked at NAS appliances, but I think it's still cheaper to create one from scratch (assuming existing knowhow, and "free" time :)

ZFS is 100% software raid.

Sorta. At the end of the day, yes, the disks are being managed by your CPU and not a dedicated raid controller... but it's a completely different ballgame than something like mdadm.

Not "sorta", it's software raid and thus, like software raid, it's portable to any system that'll run the software and JBOD the drives.

Most would argue RAIDZ is better, but many of these reasons are functionality based over and about just redundancy, i.e. Copy on Write, et all.

Friends don't let friends create RaidZ pools.

Bite the bullet of the cost of N*2 mirrors.

I say sorta because in most circles when one is speaking of 'software RAID' they are referring to a very specific configuration and are explicitly not referring to ZFS.

ZFS doesn’t support hardware RAID controllers?

It supports it, but ZFS itself is operating on the host CPU, not the RAID controller. Additionally, ZFS prefers that the RAID controller is disabled/defeated and operating in JBOD mode where disks are passed-thru to the OS without any caching.

Not just caching. The problem is that non-JBOD HBAs also swallow error codes and mask retries, meaning ZFS is potentially unaware of some read errors and is not in control of the latency as the controller firmware has its own timeouts and retry counters. Moreover, most will not pass through SMART or else fake it.

Not a Pi but the rockpro64 has a pcie port in which you can plug what you want (a sata card for example).

I made a nas with one 3tb drive and zfs. Zfs was a bit difficult to install but now it works really well and the power consumption is low.

You have to use the Linux images mades by ayufan (a third party) though. The manufacturer just list a bunch of distro that run on the cards and most of them seem to be made by some volunteer. I'm not a big fan of that.

There is nothing wrong with software raid. Synology uses SW raid in all their NAS products as far as I know, right up to the flagship models with > 10 drives and 100T+ of storage.

Hardware raid controllers are a single point of failure and when they fail the vendor sells a different product so you will always have to stock up and buy two at once.

Software RAID is easier to manage, easier to do disaster recovery, does not require specific driver support, and easier to move to other hardware.