His hotheadedness (dafuq?) He easily sometimes lose his temper when other characters pissed him about the extinction of the Kuruta Clan.
His focus while the battle is on going will be lost. :P
Kuroro taunted Kurapika and even though Kuroro didn't comply with Kurapika's demand, Kurapika refrained from killing him. Kuroro then realised that Kurapika's "weakness" is that he places a higher priority on protecting his friends than on revenge killing. Since Kurapika is tied to Gon and Killua, he's a lot more vulnerable than simply being sole operator who only looks out for himself.
In HXH you're not going to get like a 4 on 13 that becomes like 13 1v1 in a row and end up with the side of 4 never dying if the combatants are even reasonably matched. That's assuming you even have 1on1s to begin with.
For the most part in terms of killing people, Kurapika is borderilne invinicible 1on1 just due to Emperor Time. Even 1on2 or 1on3 wouldn't be too hard for him if he just immediately kills the guy he captured with Chain Jail. The other main characters don't have guaranteed 1on1 winning skills and they'll probably die or get captured and that's why it's his weakness. They could be physically stronger than him and it really doesn't matter because in HXH it's really what you do with your abilities that matters a lot more given all human characters have roughly the same base ability.
If Gon was not the main character you just hit him while he's charging up and then he dies because he's at equivalent of Zetsu state while charging up. Killua can take care of himself, but Leorio probably won't have strong killing abilities either.