The reason why people are saying that Dark type is more fragile now is probably in comparison to the Ghost type. Let's review.
Before 6th gen, Dark was weak to only two types, Fighting and Bug, while Ghost was only weak to two types as well, Psychic and Dark. Nothing else special about either type.
Then, in 6th gen, both types are able to hit Steel types neutrally. That's fine. However, now all Ghost types have a "Shed Shell" built into their typing, while Dark types gained a weakness to the new fairy type. In comparison to the extremely similar Ghost type, yeah it would seem that Dark type lost out. Dark type is similar to the Ice type in that while the actual type might be lacking, the moves of said type are not.
Now I'm not saying that the Dark type has gotten frail, and I'm not saying that the Dark type hasn't gotten frail, just that I understand where the comments are coming from.
But I digress. I like the fairy type, I just haven't really seen any Fairy type that really fascinates me (that didn't get retyped) outside of Sylveon. Although, the retyping finally gives me a reason to use Granbull over a similar pokemon like Ursaring.