First of all, i chose Flask for jamgaroo, because i haven't really done anything with it and wanted to try it out. So the choice of flask there wasn't made because i am a fan or an expert on it, but out of curiosity.
So far flask seems pretty nice. Its very well documented, it feels pretty lightweight and handles only the very basics of what a web framework needs to do. Coming from django especially the last two things are what appealed to me. Flask feels more like a python library, compared to django, which feels like a framework that uses python as its DSL. Next to that there are a ton of resources out there like libraries, non official tutorials etc.
One problem with this is that especially the non official tutorials and blogs and such on flask use their own patterns to solve certain problems and don't really follow the flask "best practices". On the one hand its nice that you get this freedom, on the other that sometimes leads to "unclean" examples and code.
In general if you want to learn it to get into python web backend dev, i would propably rather recommend django. It comes with way more stuff pre implemented you would ether use some third party plugins for or spend hours implementing your self in Flask. Also it somewhat forces its patterns on you which may lead to cleaner code and examples that are using the same patterns to implement things.