You may not be able to grow an existing vdev, but you can add another vdev to an existing zpool. And although having all vdevs be identical gives more consistent performance, it’s also possible to mix different vdevs in the same zpool. Multiple vdevs also result in better performance, as each raidz/raidz2/raidz3 vdev only has the equivalent IOPS of a single drive, but without the write hole drawbacks of more traditional RAID5. Recovery time with multiple vdevs would also be much better than a single large vdev, which is becoming more and more of a concern as drive capacities continually increase while IOPS remain more or less the same.
Yes, all very true, but all of that is utterly irrelevant for the home NAS in 95% of cases since performance is not that critical and it comes at an enormous cost of wasted space, also physical space is a huge deal since adding two redundancy drives for each raidz2 quickly adds up. As well as noise etc.
And if we want to talk optimized setups it really isn't recommended to add new vdevs to a already populated pool since the usage will be quite unbalanced and affect performance as well.
The big death blow however is the inability to grow a vdev which means you have to pay the for redundancy each time you upgrade. Which isn't cost effective, so the alternative is to buy all drives up front which robs you of taking advantage of falling prices (as you probably don't need all that space on day one but gradually increase the storage used). This forces you to pick smaller drives that are more cost effective today (compared to buying a few larger drives that will become cheaper in the future), which means that you will have to replace the drives sooner when you run out of room in your chassi (or get tired of noise / power consumption).
I love freenas, in my opinion it is the best solution for the home NAS. I use it myself, but I do consider it to be quite costly (whether it is worth it is up to you). The end results all things considering is easily more than double that of a COTS solution that you can easily grow or a more regular linux+mdadm setup. And you also need to buy more of that up front rather than spreading it out over the years.
But it does depend on what your needs are and how much data you use.