I've built and maintain several python webapps including both django and flask apps. The routing in flask is the main reason I don't start large projects in flask. This package looks good, I'll certainly be giving it a try.
What problem occurs as an application grows? I've used both Flask and Django, but have never built a large enough project to have problems with routing.
The default routing method encourages having the routing definitions ontop of the functions them define them, functions that could get quite big. It's all about coding style really, if you write your program well, it's less of an issue.