Being an Esperantist, though my interest in the language is more theoretical than practical, I used to consider the use of Esperanto as a/the family language one of the most challenging linguistic experiments. Paradoxically, the last, therefore the most basic, domain of use that persists when a language is dying, is the most difficult one to start when the language in question has no native speakers, that is, among non-native speakers.
When I participated in the annual Yiddish-speaking summer camp twice in the past, I met several young American couples of non-natives speakers of Yiddish who decided to use it as their family language. Back then I didn't think about this unique phenomenon, simply dismissing it as a rather fanatic thing. In Israel I've met a few similar couples but with a fundamental difference - either the husband or the wife is a native speaker of Yiddish.
Rather recently I got acquainted here for the first time with a couple of non-native speakers of Yiddish deciding to start a Yiddish-speaking family. I had chances to shmooze with them at length about this joint project of theirs. It was only then that I started to understand for the first time how difficult the project must be - even more difficult than the use of Esperanto as a/the family language - for a couple of reasons.
The first possible obstacle for such couples is pragmatics. Unlike Esperanto, whose pragmatics is determined by the native languages of its speakers with no single norm, Yiddish has its own unique pragmatics. One may be able to master its grammar and lexicon, but since one can't generally learn it in its still natural environment unless one has become a haredi, the pragmatics of Yiddish by its non-native speakers can be quite different from that of its native speakers.
The second possible obstacle is culture. One may be able to say that Esperanto already has its own culture, but I don't think it's culture at a different, less fundamental, level. Speaking Yiddish naturally requires far higher level of cultural literacy than speaking Esperanto naturally. The above mentioned couple I've met here in Israel has also had to learn the culture of Yiddish, both religious and secular.
I've asked this couple to keep me posted about their project. I've already decided to find and contact more non-native couples using Yiddish as their family language among fellow Jews as well as non-Jews (I know one such couple!) and compare this phenomenon with its counterpart in Esperanto. Adding similar experiments in other languages to this comparative study might be rewarding, but unfortunately, I'm not aware of such experiments first-hand. Perhaps, in Irish, Catalan, and some other previously oppressed languages in Europe, for example.