I started with Mercurial, eventually got forced into git, and now use jujutsu.
Totally agree with the Mercurial developers: Just use a branch/bookmark. When I encountered it in git, it seemed neat, but became yet another concept/thing to clean up that you don't need to.
And lo and behold, after switching to jujutsu, everyone shows how you can do a stash using an (anonymous) branch.
Even though I used stash a lot in my git days, I don't miss it at all while using jujutsu. The benefit of jj is the ease with which one makes branches (without needing to name them). That's why you may not have liked the advice in mercurial - it wasn't the solution that was problematic, but that mercurial didn't make it as easy as it should have been.
(Same goes for index - no one misses it once they switch to jujutsu).
The plugins were usually shipped with mercurial so you didn't have to install them separately, but you needed to know that you had to enable them in a config. And I beleive this turned a lot of people off.
I think some of the extensions were very basic stuff like graph logging and colorized output -- and mq like you said. So it was kind of unfortunate that people got a bad impression of hg from that and bounced off.