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