You're right, raidz expansion is brand new and I probably wouldn't use it for a few years. I was referring to adding new redundant vdevs to an existing pool which has always been supported as far as I know. E.g. if you have an existing raidz or mirror, you can add another raidz or mirror vdev to the pool. The pool size grows with the usable size of the new vdev. It's just zpool add thepool mirror disk1 disk2 as far as I know. The downside being it results in less usable space - e.g. two raidz1 vdevs remove 2 disks from the usable space, whereas Unraid-raid would remove 1. For example if you have 3x 3TB and 3x 4TB disks, you'd end up with 14TB usable space with ZFS and 17TB with Unraid. On the flip side, the two raidz1 vdevs would have higher reliability since you can have one disk die in each vdev.

just clicking a button on a webUI.

No question. I think TrueNAS offers this too.

ZFS also recently had a major data loss bug so I’m not sure safer is accurate.

Imagine how many of those would be found in Unraid-raid if it was used as widely and for similar loads as ZFS. My argument isn't that there aren't bugs in storage systems. There are, and the more eyes have seen the code and the more users have lost data for more years, the fewer bugs would remain. Assuming similar competence of the system developers, ZFS being much older and ran for production loads makes it more likely to contain fewer data eating bugs than Unraid.