Really, people should talk to Unitarian Universalists before they try to do what we have been trying to do for decades. We might have something to teach, you know. On the other hand, there are things that we UU's can learn from their experiments.
The Sunday Assemblies growth strategy is a franchise model. Local leaders apply to become part of the Sunday Assemblies, get the rights to use the name and the materials, and are expected to use them. This is quite different than Unitarian Universalists which used a locally autonomous affiliation model, where each new local group is free to shape itself however it wishes, as long as it has a democratic structure and conducts regular "religious services." Within those very broad guidelines, a new UU group can choose its own liturgy, theological perspective, and style.
A group in New York has sought the Sunday Assembly franchise but disagrees with the London leadership about where to hold services and how "atheist" the group should be. New York has broken off and formed another group the "Godless Revival". New York wanted a bar venue and a more militant atheism.
The franchise model has produced a schism where the UU model would have accepted the difference between the two groups as diversity within the single movement.
It could have been a useful conversation had they called and asked for our experience. Not that we necessarily know better.
The franchise model is better if the conception of the movement is sectarian. In UU terms, if we are trying to build a distinct UU religious entity with UU identity, it would be better if there were consistency in all localities. And it makes starting new groups easier; there is more or less a kit. A new group can be planted with fewer people and with less reliance on local creation.
If you are trying to build a more amorphous movement, then how we do things now, maybe better.
Another thing we might have warned them about: you can never be "atheist" enough. The New York group -- "Godless Revival" -- thinks the Sunday Assembly group is soft in their atheism, entertaining a broad range of skeptical views, instead of propounding a doctrinal atheism. The London leaders counseled against focusing on atheism, per se.
Atheism is a religious word, a theological position which makes no sense without theism. Therefore, to be doctrinally atheist means that the content of programming in an atheist church will be polemics against theism. This is too much religious disputation for most people.
And if you structure yourself, however loosely, on the model of a church, some people will always find it too 'churchy'. You can eliminate as many evidences of churchiness (robes, ministers, the word 'church', worship, faintly visible crosses, the color burgundy, sermons etc.) as you can, but there will always been something that triggers 'church' to the ecclesiologically sensitive. Someone will always want 'something more irreligious.'
Unitarian Universalists, if they are at all candid about their experience, could have passed this along, if asked. Doctrinal Atheism and Church Allergies are not easily contained within a more diverse religious movement.
Another lesson, we are learning, which the article showed: Someone voiced the complaint that they found the Sunday Assembly talks to be not sufficiently intellectually stimulating. I can imagine every UU minister shuddering a bit at this complaint. Intellectual stimulation in sermons is a no-win situation.
The example cited was what the complainer said was a shallow 'wikipedia' based talk on particle physics. Skillfully presented, I might find such a talk very stimulating, as I know absolutely nothing about particular physics. The physics teacher I know who makes it a point to boycott any sermon about "quantum physics and its implications for theology" would not agree, I am sure. On the other hand, I find most most sermonic summaries of US History to be too simplistic to engage me.
It's simply impossible to be consistently intellectually stimulating to a large number of people. People know too much different stuff, and are engaged in too much different stuff, with different levels of experience for it to work. They should have called; we could have steered them clear on that.
The Sunday Assemblies look a lot like Unitarian Universalism, or what Unitarian Universalism would look like if it were younger and more media savvy. I am not sure that is enough to avoid being beset by the same internal conflicts that Unitarian Universalism is.