Well you shouldn't confuse his nen-ability "Emperor Time" with his Vow/Restriction that he has placed on himself. Only reason he out preforms Gon and Killuha is the restriction he placed on himself. I usually think of it as a focusing lens, restricting the area at which you shine the light but the light becomes much more intense at the area it can shine on. Like a laser beam.
Those aren't consequences. The word consequence indicates that there is something bad by using the powers. That you would lose something for the power you gain. Non of them lose anything. They only gain. Kurapica can use his Eyes/Specialist powers against anyone. He just has problem getting the mojo going unless it has something to do with the Spiders. Shinta said that too and also mentioned a drain in power while activated. That would be a consequence. But i have seen nothing that says or indicates that he get drained while having his eyes activated. Maybe i just missed or forgot some detail that has stated this for a fact. As for Chrollo those ain't consequences either. Have you seen anyone use more then 1 power at the same time? Only one i can think of would be Kurapica that used "Chain Jail" and the healing chain at the same time but that could be argued that both are 2 different applications of the same power.
Can you clue me in on where i can find this statement that he gets fatigued for using his eyes? And the fact is that Kurapica uses all nen groups all the time. His entire chain utilizes most groups. Without Emperor Time it would be almost useless against a Spider. Chain jail that activates Zetsu is a Manipulator ability. The healing chain is enhancer. The judgement chain should be Manipulator powers. And the reason the chain is as strong and fast as it is, is because of the combined element of Conjurer, Manipulation and Enhancement (maybe even Emission). Kastro vs Hisoka showed us what happens when you focus on another group and you ain't Kurapica.