I don't know Umbran; in Dream Park, it was more action-adventure than mystery (the mystery seemed secondary, even if it was the reason for some characters' involvements).
The appeal of the novel was, in large part, the interplay between those two elements - the grand, overblown action-adventure in which the only real stakes were prestige, and the espionage murder-mystery played out alongside it for much more serious stakes. It wasn't so much a case of one outweighing the other as it was the two synergising with each other to create something greater than the sum of its parts.
I've read Dream Park, and the sequels, The Barsoom Project and California Voodoo Game. All written by Larry Niven and Steven Barnes.
It should be noted that these novels are at their core mysteries, not fantasies or action-adventure stories. They also include a goodly amount of fan-service references from the days in which they were written (so, '82, '89, and '92). And when I'm talking fan-service, I don't mean just references to media. I mean references to sci-fi fandom of the time. Filk music, for example, is featured.
It looks like there's now a fourth, The Moon Maze Game, released just this year. I haven't read this one yet.
Agreed. This may be why I didn't like the middle book (Barsoom Project). I don't remember much about it, but I remember thinking that it was just a poor sequel with nothing fresh. The third book, however, I did like, as it wasn't a rehash of the first one, and it kept the same well-written mix of crime/mystery + fantasy game in the Park.