As I'm using this more and more, another concern I have is the "touchiness" of Upfront and how it would affect clients making any changes.
I've found that any adjustment using Upfront will actually throw the entire page out of whack, and requires the designer to go back and re-establish the position of elements in the various responsive setpoints. As a designer, I'm ok with doing that when I make a change (though it really is annoying after a while and should be addressed). However, clients won't be so appreciative -- they will be scared and hesitant. Again, I'm already seeing this.
In addition to alleviating the need to access Upfront for changes by re-establishing the Appearance->Widgets tab (my last comment), I would also ask that some type of "Upfront lockdown" be easily provided for everyone's overall well being.
Also, is there a more dedicated POC for these kinds of comments. Something perhaps less public than the Support forums? Upfront is still very much in beta and I don't want to cloud actual support issues with suggestions.
I like how Upfront gives me more control as a designer and theme builder. But, there are a lot of concerns here for when I turn over a site built with Upfront to an actual user. From that perspective, Upfront needs a LOT more attention. As you move forward, I think you need to give a great deal more consideration about the ramifications of this system and how it works from a user point of view. I'm happy to help with that.