I read the update last night. I'm not a coder, but it sounds like their "building from the ground up" theme is an answer to the criticism that it had become a bit of a Rube Goldberg machine with all the piecemeal additions. As a result Umbrel lost ground to Start9, especially re personal server users. 

siggy47

bitcoin

Opinion on Umbrel Rebuild? 

Lots of UI/UX changes. Not that much of changes in internals (good? bad?). There is quite a lot of hype around. I am missing some more technical explanation of the changes. I did not study the code very deeply however.