One of my most popular blog articles is this article about the "Hidden Cost of using ZFS for your home NAS". To summarise the key argument of this article:

Expanding ZFS-based storge can be relatively expensive / inefficient.

For example, if you run a ZFS pool based on a single 3-disk RAIDZ vdev (RAID5 equivalent2), the only way to expand a pool is to add another 3-disk RAIDZ vdev1.

You can't just add a single disk to the existing 3-disk RAIDZ vdev to create a 4-disk RAIDZ vdev because vdevs can't be expanded.

The impact of this limitation is that you have to buy all storage upfront even if you don't need the space for years to come.

Otherwise, by expanding with additional vdevs you lose capacity to parity you may not really want/need, which also limits the maximum usable capacity of your NAS.
RAIDZ vdev expansion

Fortunately, this limitation of ZFS is being addressed!

ZFS founder Matthew Ahrens created a pull request around June 11, 2021 detailing a new ZFS feature that would allow for RAIDZ vdev expansion.

Finally, ZFS users will be able to expand their storage by adding just one single drive at a time. This feature will make it possible to expand storage as-you-go, which is especially of interest to budget conscious home users3.

Jim Salter has written a good article about this on Ars Technica.
https://arstechnica.com/gadgets/2021/06/raidz-expansion-code-lands-in-openzfs-master/

There is still a caveat

Existing data will be redistributed or rebalanced over all drives, including the freshly added drive. However, the data that was already stored on the vdev will not be restriped after the vdev is expanded. This means that this data is stored with the older, less efficient parity-to-data ratio.

I think Matthew Ahrends explains it best in his own words:

After the expansion completes, old blocks remain with their old data-to-parity ratio (e.g. 5-wide RAIDZ2, has 3 data to 2 parity), but distributed among the larger set of disks. New blocks will be written with the new data-to-parity ratio (e.g. a 5-wide RAIDZ2 which has been expanded once to 6-wide, has 4 data to 2 parity). However, the RAIDZ vdev's "assumed parity ratio" does not change, so slightly less space than is expected may be reported for newly-written blocks, according to zfs list, df, ls -s, and similar tools.

So, if you add a new drive to a RAIDZ vdev, you'll notice that after expansion, you will have less capacity available than you would theoretically expect. //

The overhead or 'lost capacity' can be recovered by rewriting existing data after the vdev has been expanded, because the data will then be written with the more efficient parity-to-data ratio of the larger vdev.

Rewriting all data may take quite some time and you may opt to postpone this step until the vdev has been expanded a couple of times so the parity-to-data ratio is now 'good enough' that significant storage gains can be had by rewriting the data. //

Release timeline

According to the Ars Technica article by Jim Salter, this feature will probably become available in August 2022, so we need to have some patience.