I've used both Flask and Django for apps that are both simple and complex. What I have discovered, is that if I use Flask for a complex app, I need to organize my code in clean components that end up looking like Django. Yes, I can do this more-or-less in a freeform manner with Flask, and even use the "one file for everything" method, but for a complex app, I wouldn't want to. It's too hard to manage.
So, in the end, I end up mimicking the basic structure of a Django project: Models, Views, Forms, Utilities, etc. Yes, different ORM, different Form library, WSGI middleware instead of context processors, etc., but all-in-all, no less complex than had I used Django. It's really to the point that I find it hardly any faster to code in Flask for all but the most basic sites.
I still use Flask for super simple projects that require no Admin interface, such as Personalized URL sites for ad campaigns. So it certainly has it's place.
MVC is a local maxima we found to build complex apps. Complex app will always require complex structure no matter the environment.
I don't want to disagree on something I don't have much experience with, but at the same time, I'd like to hear more about how you build your projects.
(In addition, considering there are two viable options for plug-and-play Flask admin interfaces, I don't know why you made it sound difficult.)
I'm not saying it is more difficult. I'm saying that it's not any less complex by the time you put everything you need together. In other words, it's a wash, so I just use Django instead. Fewer dependencies to install, fewer helper functions to write (because many of them already exist), and more components that are designed to work together.
And then there are other things such as having all of your URLs specified on one place vs. as decorators with your controllers. But that's just my personal preference.
You can sort of maybe use SA with Django, except when all the apps you use depend on it and you have to use SA reflection to get at your data. Sadly, the integration is poor.
I totally agree about the regexes: only a small subset of all routes will ever need regexes, and you can do it explicitly in Flask when needed.
SQLAlchemy has pulled so far ahead of the Django ORM in features and usablility that I hope with the Django 3.0 release they replace their ORM with SQLAlchemy. For all the basic use cases it is pretty easy to add in the Djangoisms to the SQLAlchemy ORM layer.
I don't have the time to organize my thoughts but I would like to pipe up here and say that the it's not fair to call the Django ORM 'terrible'. It's extremely easy to learn and does a good job for a very large chunk of plausible use-cases.
But it does an absolutely terrible job for another pretty large chunk of use-cases, mostly because it's closer to objects than relations.
For example, Django's ORM has no concept of explicit joins for example, which are necessary for avoiding O(n) # of queries in several cases. The only way to do it is to write SQL, whereas with SA the query is trivial to write in Python.
That is not true it is just that the syntax is non-obvious and awkward in my mind.
You can do stuff like Teacher.objects.all().select_related('students') and Teacher.objects.filter(students__first_name='Jill').
The biggest frustration is aggregation though. It has hard and sometimes impossible to add group by and having clauses. Using extra() is very fragile and using values to force the group by is awkward. There is probably no fixing that part of the Django ORM other than just getting rid of it.
There are still joins that are not expressible through its syntax, like outer joins between several unrelated models.
I've been pondering implementing a Django ORM backend that just delegates to the relevant SQLAlchemy functions. I think it may be way harder than it seems, though.
This is pretty much exactly what happened to me. I started building parts of django in Flask in my own weird way (probably complete with anti-patterns etcetera) so I thought why am I doing this when there is a team of highly skilled people dedicated to making another system to do exactly this? So now I use Django. And it is AWESOME.
So, in the end, I end up mimicking the basic structure of a Django project: Models, Views, Forms, Utilities, etc. Yes, different ORM, different Form library, WSGI middleware instead of context processors, etc., but all-in-all, no less complex than had I used Django. It's really to the point that I find it hardly any faster to code in Flask for all but the most basic sites.
I still use Flask for super simple projects that require no Admin interface, such as Personalized URL sites for ad campaigns. So it certainly has it's place.