There seem to be different opinions on how similar the members of this distro-family are, and whether a user of one distro can support an user of the other distro. I have read through some discussions here on AskUbuntuAsk Ubuntu-Meta and can understand both sides: One that says "under the hood, they are the same" and the others "they are configured differently" (there are other arguments as well, but for me most of them are less technical and more political).
The fact is: If I have an issue with my Mint, I enter the error message into my search engine and often end up on a Debian or Ubuntu forum, e.g. AskUbuntuAsk Ubuntu. And yes, in at least 90% of the cases I can apply that to my Mint. I bet you have had similar experience, with issues on your Ubuntu being solved on forums for other members on that family. Please don't wonder about How do Mint users end up here?, under the hood, all Debians produce similar error messages.
But of course, there are differences! And I understand that some users on AskUbuntuAsk Ubuntu consider e.g. Cinnamon questions off-topic. I would consider YaST questions off-topic too. But do you really need to reject all questions that mention that magic word "Mint"? Feels like some people have a Mint-allergy.
Coming to an end, what are the relevant differences between unsupported derivatives like Mint and officially supported variants of Ubuntu? What kind of questions can a Mint user ask on AskUbuntuAsk Ubuntu (because a Ubuntu user may come up with it too), and what kind of questions shall he/she better ask on e.g. http://unix.stackexchange.com/? Can we make a list of that? E.g. all questions related to the desktop are off-topic on AskUbuntuAsk Ubuntu, all questions related to libs, drivers, kernel, ... can be asked on AskUbuntuAsk Ubuntu.
I didn't ask whether Mint is officially supported by AskUbuntuAsk Ubuntu. I asked about the technical differences, and topics that can be asked and answered for either Mint, Ubuntu and other derivatives as well because they share the same technical basis.