That's a huge subject of debate. "Sometimes?" PoW is very unsecure for any chain that is not the dominant hashrate chain for their algo, for example. Cheap & easy to do double-spends via a 51% attack for example a la vertcoin, Eth classic, etc. But both are at risk to having a large group of validators owned by a single or coordinated group to break BFT. This is why Telos says each validator must disclose ownership and cannot have cross-ownership. I don't know of any other chain that does this.
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