One of the features I most disagreed with in Rails and regret in Formtastic is nested attributes. I believe they create complex coupling between the form and it's underlying models that's unnecessary.
Best practice is to build a "presenter" model that presents the multiple underlying models as one for the purposes of the controllers and views, so I feel this should be encouraged and eventually enforced in Formtastic.
Before this, we would need to provide a really solid example (perhaps a "model" that can be inherited from, if no suitable libraries exist?).
My issue with this is that every beginning rails tutorial uses nested attributes, so not supporting them is going to open a bunch of "why no support for nested attributes?" issues. If rails deprecated support for it in say strong parameters,then I'd say fine, but as it is I vote for keeping the support in formtastic.