each UI wants its own asset compilation settings.
Strange, I’d have guessed that sharing assets is more likely than having separate assets for different domains (I’m currently doing this, three domains at three different endpoints in the same VM, all sharing assets).
Counterexample: the apps that make up the OTP runtime are individually upgradable and versioned - and happen to organize the code! For instance, here’s the app file for :crypto
Some of those ideas were directly added in 1.11 - PR and announcement
You have to say “I’m on Windows” too, but that doesn’t mean everybody should stop using it. 
I run on 1.11, and that doesn’t solve the same problem, application boundaries are not the same as domain boundaries, and I believe that when most people reach for umbrellas, they want to solve the latter, not the former. There’s no reason to use umbrellas when just defining top level namespaces do the same, and don’t break tools like mix xref.
At least it’s not a problem the language itself is introducing, cross compatibility is a known problem in most software even nowadays.
There are also some other issues I’ve ran into, like having to force-compile the whole project because the order in which the sub applications were compiled somehow broke the whole umbrella, but I’d have to dig more into that bug again to provide a reproducible example(it had to do with running the project sometimes from root, sometimes from a sub app). I know for sure that wouldn’t have happened if the project wasn’t an umbrella, because it was precisely the “this module isn’t available because it’s in a different app” error.
I may be wrong but perhaps he meant that each UI/app needs different assets building tools such as webpack and so…
That’s cool but doesn’t address the problem that running mix xref at the root of the umbrella basically doesn’t help you with anything that you would want when you normally run that command. 
To me umbrellas are kind-of sort-of like the central Docker Compose / Kubernetes config file for a project that spans several microservices.
From what I’ve seen while working in Elixir companies, umbrellas are often liberally utilized as a stylistic / personal choice (usually by the CTO’s preferences) and are rarely used as they are technically meant to, leaving some of us to deal with the aftermath. Which is, again, utilizing mix xref to understand dependencies in a project better.
Having mix xref not work well in umbrellas leaves me with one less tool to enforce the single responsibility principle. Happily boundaries came along. It and mix xref – outside of umbrellas – have measurably improved the quality of my work (IMO).
Not saying everyone should be like me – but I personally will go out of my way to make a dynamic language a tad more predictable and easy to reason about upfront. Umbrellas stand directly in the way of that goal and that’s why I avoid them.
I have this non-umbrella app with two Phoenix endpoints, “api” and “web”: lib · trunk · Mikko Ahlroth / minisome · GitLab – And it really wasn’t any hassle at all. They’re both completely separate, yet in the same project. Now only one of them has frontend stuff, so setting up esbuild/webpack for two endpoints would have to be a custom job, but the Elixir side was easy to have separately.
A few months ago I started consulting on an umbrella project (about 10 apps). The first thing I did was deumbrellization of the project, which brought some benefits:
As an example of the last point, various paths that are printed by mix tasks (e.g. test IIRC) aren’t “clickable” in vscode (I couldn’t click to open the file in the editor), because the printed paths are missing the apps/myapp/ prefix. I found this extremely annoying and disruptive. This alone is for me the reason to avoid umbrella as much as possible.
Generally, my sentiment is that if the subapps are not deployed separately, the umbrella app doesn’t bring anything useful to the table, compared to single project + boundary. Even if boundary is not used, I find the benefits of umbrella don’t justify the issues mentioned above.
Thank you for sharing your settings. I will probably steal some ideas from it for my future projects. ^^
My concern is that if I use customized folder structure within a Phoenix app, I couldn’t take full adavantage of Phoenix generator anymore. The generators I think expect a given structure.
For example I don’t know how I can tell mix phx.gen.html to put generated controller-view-templates files sometimes in admin_ui folder and the other times in the default_ui folder.
Another concern is for example I would like to have different priv folder per UI for gettext translations instead of using a single folder with domain translations. Consider that my UIs don’t support the same set of locales and have their own domains of translation as well.
If there are a number of generators that don’t support umbrella project, I also feel like many can work out of the box without even being aware that the project is an umbrella. After all an umbrella app is just an app like another one in the first place.
I think it is a mater of preference but I’m really happy with umbrella as long as I keep the number of apps small and of course make sure all my apps depend only on the core app.
Thanks everyone for the discussion and sharing experiences. As I say, all the large projects I’ve worked on have been flat and have only experienced umbrellas on small (mostly personal) projects.
Much food for thought. I think the most significant issue with umbrellas reported here is around tooling, which is also an issue that I’ve also encountered. (There’s also that example and tutorials are written for flat structures and need to be tweaked for umbrellas). I really get how that could be a deal breaker.
I’m also inspired to take another look at boundary, which looks pretty useful. (I think the last time I looked at it things were in a very early state and I didn’t get round to investigating it further).
I’m curious about how much faster.
Amen.
I wonder why you did not report a bug (or why it was closed when you did) as this simply seems like a bug to me.