You can have disks of differing sizes with ZFS, though you are making things difficult for yourself so your point still stands.
However the cost of ECC is not negligible because you need your CPU and motherboard to support it.
The total budget for my NAS was 1000 EUR in 2013, 500 for the 5 3TB disks, 350 for the motherboard+CPU+8GB ECC RAM, and 150 for the case, PSU, system SSD, and accessories. In reality I salvaged 2 3TB disks, lowering the cost to 800.
By using non-ECC I could have used a cheaper motherboard and CPU, in addition to cheaper RAM. In fact I would probably have used hardware from an older desktop PC. It would have been a 15-20% saving, or 45% if I take reuse into account. Not negligible.
My previous NAS, running linux soft-RAID, entirely made of salvaged parts except for some of the disks had a few corruption problems. One of them caused by a defective disk. ZFS would have caught it, so even on cheap systems, ZFS has its use.
I also had defective DRAM, rebuilds not going smoothly, etc... That system caused me too many scares, so I decided that the next system would be cheap but not too cheap as to endanger my sanity. I also got a proper backup solution.
However the cost of ECC is not negligible because you need your CPU and motherboard to support it.
The total budget for my NAS was 1000 EUR in 2013, 500 for the 5 3TB disks, 350 for the motherboard+CPU+8GB ECC RAM, and 150 for the case, PSU, system SSD, and accessories. In reality I salvaged 2 3TB disks, lowering the cost to 800. By using non-ECC I could have used a cheaper motherboard and CPU, in addition to cheaper RAM. In fact I would probably have used hardware from an older desktop PC. It would have been a 15-20% saving, or 45% if I take reuse into account. Not negligible.
My previous NAS, running linux soft-RAID, entirely made of salvaged parts except for some of the disks had a few corruption problems. One of them caused by a defective disk. ZFS would have caught it, so even on cheap systems, ZFS has its use.
I also had defective DRAM, rebuilds not going smoothly, etc... That system caused me too many scares, so I decided that the next system would be cheap but not too cheap as to endanger my sanity. I also got a proper backup solution.