TBH the issue I've had is a combo of what Imagination said (drop outs), and the simple restrictions of turn by turn posting. If two people are online at the same time and want to play...they can each only post once because the convention seems to be everyone involved in the game posting once then starting again with person 1. This is so restrictive to me, and it make moving things alone quite difficult if you're waiting on one player who's sick/busy/ect. It seriously increases the number of dropouts/dead games, IME.

The alternative is random posting, although in advanced this doesn't seem to be done much. That can lead to speed posting, where two players advance themselves, but the rest of the group is left behind or out of the loop.

I'm more used to playing on email groups, where you can play with many different people essentially as a 1-1 or small group, at the same time within the greater confines of the game, or games that have an entire forum to themselves where you can create new topics and some can involve everyone, while others (occurring at the same time) are 1-1. This means you can play more often, interact more specifically with other players, and the active people are satisfied without having to wait for the slower posters.