[vetinari]

Just because there's a property doesn't mean that it works. It floats around on the internet, because that's the experience people have.

[rleigh]

It works just fine. I've upgraded vdevs this way several times, and it worked without a hitch on every occasion.

[vetinari]

It didn't for me; in the end, I backed up the files, scraped the machine and put the drives into Synology box.

So take it as an piece of the puzzle why is Synology more popular.