Not sure why they'd write all these libraries from scratch instead of just reusing something like Play or Dropwizard that are more used and thus probably better supported and have more of the kinks worked out. Wish they'd explain that up front.
I know you don't like talking about it, but do you have something to share about its performance?
I'm not saying only play has this problem, almost all java frameworks from the time of play have similar properties.
On the other hand, it is slow, embarrassingly slow (try running it on RPi). For me it's really complicated as well. You have routes file (which is redundant). Templating language forces you to write Scala which then uses a Scala compiler to compile which it is really slow (even Martin said it's slow). It's a vicious circle.