I've used SqlAlchemy with Flask. You only need a few lines of code to integrate them.

But yes, with Flask (or similar frameworks/libraries), you need to do the integrations yourself with the components you want to use. Advantage of Flask in this regard is, you don't need to fight with Flask to make it happen. That's why I'm saying it's better suited for complex apps that you want to build with your own abstractions.

The real problem is the cargo cult thinking when it comes to frameworks, instead of doing a cost/benefit analysis and thinking about trade-offs.

Following statements like "Django for complex apps, Flask for 100 lines of apps" without researching the reasons behind probably will result in more problems in the future.