I think that the main reason why they banned shadow tag is to prevent setup, as gothitelle's Evolution line has access to great setup moves and would otherwise take advantage of defensive mons and sweep to victory. On the other side, dugtrio and the more gimmicky anchor shot Dhelmise lack those setup moves and in dugtrio's case, it even lacks survivability, the fact that it only ever runs a focus Sash, reversal sets really speaks to that, a reversal that does pretty lackbuster damage against anything remotely defensive. Should they ever received any noteworthy boosting moves, they would probably be banned. But dugtrio's horrible bulk means that prediction+Stealth Rock means instante death, and if it avoids it, it is only trading pokémon, because he isn't pulling Off a sweep any time soon. Thus actually making them fairly balanced on the whole.