BU supporters' understanding of consensus systems in general is completely wrong, not just decentralised ones. I don't know why; it's a simple concept: it means that several independent implementations of a program given the same input data will give the same output, and those that give different results will automatically be known to be wrong. In the specific case of Bitcoin, it means that given multiple conflicting transaction chains, of which only one of them can be valid, all Bitcoin nodes everywhere will always agree on which chain is valid.

BU's so-called "emergent consensus" is not a consensus system of any kind. It explicitly allows nodes to disagree on which of multiple conflicting chains is valid, and it's trivial for malicious miners to force a chain split among disagreeing nodes to allow double-spending. It's not even fair to call this an "attack" since this catastrophic breakdown of consensus is by design. Anyone supporting BU deserves to lose their coins, and I think they're likely to.