Google groups was based on Google acquiring DejaNews and building a usenet archive and interface on top of that - mailing lists are a later addition. The usenet interface was (and I guess still is?) terribly broken, making it a pain for other usenet users to deal with posts from google groups users, because it simply works in a way that hinders interoperability with the rest of usenet.

(And while at it: The mailing list feature is also quite defective, obviously built by people who don't really have a clue how email works.)

OK, I understand what you're saying. I'm not really familiar with the history of Usenet and Google Groups it seems.

And for mailing lists it could be better, but as a user of several mailing lists of various open-source projects, I have had no problems.

I am noticing that Google doesn't work in our interests though. For example I am disappointed by their shitty support for IMAP, CalDav and CardDav.