I do agree a lot of them don't have bad stats. It's more along the line of sometimes they're only used because they were fairy. If there was a bulkier mon with better defensive ability that can be Fairy, they would have chosen that one.Fairy types do not have bad stats at all my dude. The reason tera Fairy is so good is that the Fairy type itself has weaknesses so insanely niche that the types which are super effective against it are not super effective against much else. What actually genuinely carries Steel or Poison moves, outside Steel and Poison types, unless it's to hit Fairy?
Then factor in you can just not swap into those extremely niche weaknesses, and that's why it's so good.
And as you mentioned, some Pokemons are even better with this because they naturally resists Fairy coverage (Dirge resists Steel and Poison). So the idea was that if Fairy Tera was banned, then these "problematic" strategy would be largely nerfed. Personally I don't believe Skeledirge is the worst offender of Tera, I am just drawing this picture because it's easiest to see why some typing can be exceptionally stronger than other Tera typing.
Expanding on this idea, other common "problematic" typing would be Tera Normal, for ExtremeSpeed users like DNite, or Population Bomb Maushold. Tera Flying for Acrobatics Energy Booster Roaring Moon, Tera Dark for Roaring Moon and Chi-Yu and Chien-Pao, etc. Banning these Tera types also hinders some other would-be abusers for example, and also boost the diversity of the meta because people would get more creative with Tera.
P/s: I am not saying very complex ban like "this Pokemon cannot use this specific tera type" but more blanket Tera type ban. We can already see Normal, Dark and Flying are very good offensive typing already, and Fairy is a good defensive typing, which means they're clearly better than say, Tera Bug.