I switched to raid z2 from a 6 drive mirror and what an ordeal that was. It's because I had to grow into it and buy drives over time but eventually the mirror was too inefficient.

I moved data around like 5 times all because I still didn't have enough disks to build my new array and keep my data on the system at the same time. And expanding raidz expands parity on all disks but not the data so you have to recopy all your data so it stripes fully.

I had a backup on a DAS but USB is slow and I didn't want to have it be the only copy.

Edit: clarifying my point. I have no regrets. ZFS is awesome. But make the important decisions up front and yes start with the right amount of drives that you need. My whole issue was growing into it and having to make changes after the fact.