You're about to read a dissenting view.
Yes, there's much that Sunday Assembly (and another US-based movement, called the National Oasis Network) could learn from UUs. But I would contend that the relatively rapid growth of these so-called "atheist churches" also shows that UUs might learn more from them. Such as ...
- Keeping the message simple: The Oasis Network holds to five basic principles, beginning with "People are more important than beliefs." Sunday Assembly's philosophy is expressed even more succinctly as "Live Better, Help Often, Wonder More."
- Including without diluting: Both of these networks were started by committed secularists and atheists, yet they also welcome and include the full range of people who would not fit into a traditional religious communities. The emphasis is less on bashing religion (like the "New Atheists") or mimicking it, and more on building an alternative community around shared ethical concerns and psychological needs.
- More egalitarian: Linked to both their inclusion and freethought roots, the communities in these networks are less reliant on professionals and experts, even drawing on models from Quakers and more radical "emerging" church groups.
- Meeting people's needs: Atheist and Humanist groups have previously focused on intellectual needs, almost to the exclusion of emotional and aesthetic ones. Sunday Assembly and Oasis are attempting a more holistic approach.
People will find ways to meet their needs, even if it means sidestepping "official" channels. I've seen it in my work as a medical equipment specialist, where families are willing to pay out of pocket for walkers or electric beds rather than wait for doctors to fulfill the cumbersome requirements of Medicare, or endure the waiting lists of our competitors. When you need a wheel, you find a way to get one, even if the only "legitimate" supplier tells you to fill out a form and wait for their staff to get to it. Same thing for the kind of "alternative" community offered by Sunday Assembly, Oasis Network, or UUs.
Where the efforts of these newer groups will lead, I don't know. But my impression so far is that Unitarian Universalists will not learn from what success they've garnered so far by ignoring or dismissing them.