Exactly my point proof of work can not be Sybil attacked, and under Bitcoin Unlimited the blocksize limit is determined by the economic majority through proof of work. Not the nodes like you claim, therefore BU is not vulnerable to such an attack vector.
BU nodes do not accept the longest chain if it disagrees with their idea of validity (e.g. block size), until that longest chain is a sufficient number of blocks ahead. This creates forks that will persist for up to an hour or even longer, and can be used for attacks. Many merchant systems currently use 1-3 confirms on Bitcoin and will either have to change that to a higher number or be easily attacked.
Yes, that's a good point
There are situations where you should wait longer with bu to be sure a tx is really confirmed. But this only happens when the First block your tx is in raises the size.
If You receive tx you should set your limit very conservativ to ne sure you are not on The wrong chain.
Every miner doing this risks to loose his block, so the theory is that this kind of active consens seeking will rarely emerge.
Sorry, typos, Smartphone ...