I have both, when I have to work on dolphin, I have the impression that the time machine really exists. What Andrew explains in a few words looks simple, but for whom has followed all the story, in fact, the work done is really gigantic.
By cons, yes you're right, the app market is much poorer, it's a fact too, it's almost the only weak link compared to dolphin and other "cms". That's why I think it will necessarily develop soon.
Documentation question, well I think that UNA is well provided in relation to Dolphin. It is true that the learning of UNA is longer because there are so many more possibilities. Can always improve with time.