ORIGINAL: Deja Bru
Full disclosure: I am a moderator over on the Slitherine OOB forums but don't let that concern you. I am no fanboy; just a customer with an inordinate amount of interest in this game.
As to multiplayer and PBEM, I cannot comment. I play exclusively against the AI. I can tell you, however, that the game itself is sound and well-designed for the most part (current version 6.1.9). Most of the issues encountered these days are lingering minor faults in unit characteristics and scenario design.
Designing a scenario for single play versus the AI is tough, take it from me. It requires many more "triggers" (programming entries, essentially) than a multiplayer scenario does. Each trigger introduces the possibilities of conflicts and errors.
That said, I am satisfied that the DLC and the custom campaigns and scenarios are generally well-designed and functioning well. Things crop up once in a while and are reported in the Slitherine OOB forums (not as much here; this forum is exceedingly quiet, probably because people migrate to Slitherine where the action is). The DLC issues get fixed but not until patches are released. The custom stuff gets fixed if the designer is still around and engaged. Erik is definitely one of those people.
Lastly, but certainly not least, a customer need only purchase one of the official DLC to have access to the custom campaigns and scenarios contributed by members of the community. That is a big, big plus.
Note to self: Lose this Deja Bru persona. It is confusing as heck. Note to community: It's a long and old story. This is the real me, bru888.