...: simple rules, fast play, genre flexibility - is this not a GM's dream come true?...
It is. It strikes the fine balance between being coarse enough to be easy to run while fine enough to be engaging to play.
Similarly, I did a D&D Miniatures mini-campaign using the advancement rules from the Miniatures Handbook. It was fun, but I think the players felt there was not enough for them to do with their characters; which I interpreted as not enough player investment because there was not enough work for the players to do on character creation & advancement. Coarse enough for me to DM but not fine enough for the players (see above). The latest Gamma World had similar issues for us in play (see more above).