I think flask is great, like sinatra, for small apps. But I would never consider it for 'big' apps that represented more than half a dozen api endpoints and some sort of in-depth feature set.
Above that size, unless it's super simple, you'll pay the price in tech debt. While our api has worked up until this point, we're gonna scrap it for a Rails api-only app because we need convention + orm + other things to scale not just the app, but the team. There probably is a way to do this in flask, but 95% of our team are Rails guys, so it just makes more sense to go with q company wide maintainable system vs. what's there now.
For small/single devs working on small projects, flask (and sinatra) are great.