Yes!!! I have this entire theory (are you surprised? :-) about long stories and entering fandom (esp. when you're not that familiar with the source text yet) and it relies exactly on that same idea. Longer stories are self-contained in a way that shorter ones can never be, not just in plot but characterization as well, yes!

I'd never rec even the best short to someone to get them interested in a new fandom.

And personally,I have this bizarre AU fetish, b/c to me the central appeal of the characters comes out best when I read a bunch of good AUs...short stuff takes so much canon detail knowledge b/c it tends to rely more heavily on intertextuality to be comprehensive, not just too add yet another layer...