Both points you make are certainly valid. I'd say that Isso really targets low readership blogs that do just fine on a cheap unmanaged VPS instance though, so a

Both points you make are certainly valid. I'd say that Isso really targets low readership blogs that do just fine on a cheap unmanaged VPS instance though, so a high availability alternative isn't a requirement for that demographic. Unfortunately this also rolls over to the spam filter portion. There were discussions in the bug tracker about this a while back, where the main Isso dev stated that it's a mostly obscure piece of software and spammers are not currently targeting it, so why bother worrying...