KYC is stands for "Know Your Customer" and i think this is a system that identifies the true identity of a participants. And this KYC is important in order to validate cheaters and those scammers. But sometimes it can be used also by those scammers also by pretending to have a n ICO and will ask your information for their own benefit like using your information for bad intention and this is the disadvantage of KYC system implemented.
This is a simple yet a good explanation how "know your customer works" . They need so that we can first know their customer very well and second for the security of their customer. That we can guarantee that our tokens are safe to be distributed to us.
I dont think there is a disadvantage in kyc system. It helps customer a lot.