I'm no Dancer expert and I only have limited experience with using it. But that was still long enough for me to experience ETOOMUCHMAGIC from Dancer. Since then I have repeatedly seen evidence of more of that. I also see Dancer/2 suffering more from "framework disease" than Mojo does. Framework disease is where there is much rejoicing about how you can build a whole functioning system with just 1 trivial line of code. This then leads to major spelunking required to detangle all of the magic that was hidden in order to make all of those things dance with such little specified. The spelunking need can come up any time you want to do something even a little different than the default way.

But then, the many things I built didn't even use Mojo. They just used Plack.

As for building URLs with concatenation, I've just come to expect such stupidity from all web frameworks. Before I had experience, I expected building URLs properly would be something built directly into JavaScript. Then I expected it to be someplace obvious in jQuery. Then I expected Mason to handle such things. But I almost never find tools for doing that in such places. And I have never found code written by others and using such tools that builds URLs other than by concatenation, and only rarely is any escaping done before concatenation.