Any proof-of-work based system has this problem, especially when the PoW is outsourceable. However, I believe you are missing the point of Byzcoin, which is that if there is a set of decentralised miners we can still get the strongly consistent, final guarantees of Byzantine agreement in a scalable way.

ByzCoin has PoW to be Bitcoin compatible, but it can change to PoS or PoA or even permissioned systems (e.g hyperledger).