Smokemonter (and few others) oversee the administration of the github for the jailbreak but have nothing to do with the jailbreak implementation.
What are the garbage MiSTer cores? The popular ones (NES, SNES, Genesis, TG-16/CD, GBA) are pretty much flawless at this point and to be honest the equivalent released Analogue cores have some catching up to do.
GB, SMS, and Neo Geo are all excellent. It sounds like your impressions might be based on info from two years ago, but a lot has changed since then.