In 5 years I've had 2 family support calls: one to setup a new WiFi on a Chromebook and one for turning the Chromebook on and off.
This clearly invalidates your point, and supports "Linux would be either painful or more realistically a no go", because it's obviously not enough for some relatives to be able to easily use Linux - most of them need to be able to. The fact that you have more capable-than-normal relatives or more Linux-friendly-than-normal hardware is irrelevant to the fact that most people do not.
All he needed was a web browser, an email client and a word processor. And that's it. So Lubuntu did the job perfectly.
Obviously more expectations may be harder to meet
It's not enough for some people to be able to use Linux - if you want to recommend it for general use, it has to be usable for almost everyone.