I would need to see some replays of this in action, because I can't see how dark's defensive core deals with the enormous power creep in the tier so well as to be considered "broken".Sorry if i'm causing headaches to the monotype OMs "council" but I would also like to adress another issue, namely sableye in STABmons.
Sableye is a fairly common defensive pokemon in stabmons, who, with the combination of prankster and the skills topsy-turvy, parting shot and recover + filler is an excellent addition to most stall teams. However, since monotype restricts you to using a single type when creating your team, this causes problems in teambuilding for most types, as it is practically impossible to get through the dark defensive core for all types that don't have access to either a strong dark type mon (to resist prankster), or fairy, which can hit most of the dark defensive core super effectively. In fact, without some serious shenanigans like white herb or z stat boosts that negate negative ones, i have a hard time seeing other types get through the defensive core at all, much less be able to snatch a win. Therefore, I would nominate sableye in STABmons as a possible ban target, and would love to here other people's ideas on how to deal with sableye and the dark defensive core (tyranitar, alolan muk, buzzwole) without overcentralizing their team, with all types of course.
The lack of sturdy water resists is the most obvious problem; Keldeo, Kingdra, Araquanid all pose a tremendous threat to this core. Rock spam is a threat as well; with threats like Mega Tyranitar or SD Terrakion bopping the whole team. Even when at -1 threats such as Band Head Smash Aerodactyl 2HKO the entire defensive core. This isn't limited to just Rock or Water teams either; unless your team is obtusely reliant on set up sweepers I can't imagine why it couldn't beat the defensive core, assuming it's well built.