Flask would be more comparable to RoR than to Angular, since it is principally for backend work. (You can do server-side-rendered front-end stuff with it, with templates, but I choose not to -- currently working with Flask for my backend API, React for frontend).

There are two main features of Flask for me, over something like RoR: it's Python (my preference over Ruby); and it's a very light framework with a ton of modules, compared to my perception that RoR is pretty big and heavy and opinionated.

YMV, I only do this stuff in my evenings so I'm not terribly familiar with it all.