difficult for me to say what is the more difficult things to understand in PureMVC now that i understand the main framework "rules"...
But,I think that for all stuffs we focus on, we have to work hard to understand all the concepts and best practices.
The first thing that comes in mind is (as for other frameworks) to understand that even if we have first to make a lot of codes for little tasks, it's to make the applications more robust and extensible...
After that step, i think that (when understanding main design patterns and other frameworks like Cairngorm) PureMVC give us in two packages (one core and multicore), all the architecture and tools we need to make our life easier !
When i first began to work with pureMVC... while i was progressing in my development, i found all the answers i need to continue (utilities, forums examples..) .
The support forum is great and there are a lot of basics and more complexes examples to learn.
Authors are always available to answer our requests... It's very motivated..
So.. What makes PureMVC a great framework is that when we put our hands in it we found things naturals, everything seems more clear and more "real life" oriented in terms of communication and architecture.
In conclusion, i think that Jason is right in the "Hardest" concepts to learn , and i think all the utilities are essential to realize the real benefit of PureMvc.