I would question just how 'overpowered' these specialist abilities are. I haven't seen an ability that is inherently better than the more generic ones. Neither Chrollo or Kurapika's abilities particularly impress me all that much.

Sure Chrollo can copy nen abilities but he has a lot of strict rules to adhere to.

Similarly, Kurapika may be able to temporarily use all nen types at 100% efficiency with his eyes activated, but he would still have to spend quite a bit of time training these abilities. Since he gets fatigued from using his eyes too much, it does not make much sense to prioritize other nen categories over his own. Emperor Time may have helped him out in a pinch against Uvo, but ultimately it was his Chains that allowed him to triumph. Also, if the fight between Kastro and Hisoka showed us anything, it was that straying too far from your category is a recipe for disaster.

I feel that the Enhancer, Emitter and Transmuter categories are more well-rounded. If anything, the affordance given Conjurers and Manipulators to become Specialists later on probably balances thing.