The main problem with ChromeOS Flex is it's limited hardware support. I installed it on my mum's old Dell Optiplex as a trial, and whilst it worked fine, it didn't support her Canon Pixma printer/scanner. I looked it up online and others had the same issuse as well, and Google said unfortunately there was nothing you could do besides getting a compatible printer.

The second problem is that ChromeOS Flex has a limited support period. It's no doubt longer than Android, but it's still artificially limiting the life of older hardware that has potentially still many more years of service left. Like my mum's Optiplex 9020 which reaches end of support in 2026 - whereas I believe it's got at least 5 years, if not more, especially considering that I could upgrade the RAM/CPU/disk if required.

The third issue is that it's Google. There's no telling when they'll pull the plug on this.

The "PHD" solution I ended up going for was Aurora (an immutable distro based on Fedora Atomic). Aurora uses KDE, which was simple and familiar enough for my mum to use with almost no training, and its got atomic automatic updates with the last two images also available in the boot menu, in the rare event something goes wrong. It's practically zero-maintenance like ChromeOS, with none of the drawbacks.

Oh and in case Aurora ever decides to fold, it's easy enough to rebase to another Fedora Atomic variant with just a single command, without affecting your data.