Hmm, I think this post + the microservices one got me thinking.
Flask is very convenient with the built-in session dictionary. It's hard to say if/when I'll switch to microservices for one because I then have to take on that extra work manually. Using Flask, we can pass along some session info inside the {{ }}. However, Angular interferes with this, and at first I was thinking I'd need to set up a microservice to send JSON to Angular and deal with cookies/session/auth for every call. But maybe Flask-Triangle solves this problem.
I can send the {{ session info }} in the Flask template, then set that to an Angular variable and still use it with a notation like {{ session info | Angular }} so that Flask ignores the second thing in braces. No need to send JSON. The app is then more monolithic, for better or worse. Hmm. Edit: at least for some basic info, that seems ok.