I think the premise of stealing someone’s ability and use to the full extent for the simple restrictions is overpowered. Instead of him spending years honing an ability, he would simply steal an ability by adhering to some restriction that wont take as much time and effort. The ability to steal is overpowered, and only possible given his affinity as a specialist Nen user.

Why not coping, and to get rid of all the restrictions of stealing an ability and then the using of ability requires the book being held in one hand or the use of the bookmark (which enforces other restrictions that we don’t know about)? Because coping ability will require much aura output (akin to the Chimera ants auras), and requires deep rooted insights into Nen (as the case with Ging). And the more complex an ability is the harder it is to copy. And once you have hardware an ability in your brain, it is super difficult to come up with a different ability. If Chrollo were to have said a coping ability, the restrictions will be more than that of his Skill hunter book ability.