Current situation: blocksize cap for the network is set by miners/nodes converging on either Core, XT or something else by downloading and running Core, XT, etc.
... and making the decision not to modify this code.
Yeah, and it's worth noting the reasons people don't usually mod the blocksize in Core: because there are overwhelming economic incentives for a miner not to mine or build on a block that will likely be orphaned, and overwhelming economic incentives for node operators doing business not to accept such blocks.
The same incentives apply to BU. There are [repeating the above verbatim] "overwhelming economic incentives for a miner not to mine or build on a block that will likely be orphaned, and overwhelming economic incentives for node operators doing business not to accept such blocks."
As far as I understand it, BU merely removes the artificial authoritative "oomph" of the blocksize cap setting being hardwired in Core (and XT). It can be made to default to Core's blocksize cap settings (it doesn't now, but it would be trivial to make a fork that does), and there can be scary warnings about changing it away from Core's settings. It just takes away the inconvenience barrier, with the hope that people would follow a different Schelling point than the one set by Core. Failure of BU probably means just "everyone sets their settings to mimic Core." That's about the worst it could go, provided there are no coding errors (community definitely should vet it carefully; better yet, Core could just make the blocksize cap user-selectable).
User selectable cap means sybil attack. Again, nobody is answering my questions here.
What will BU do to stop my 2000 nodes acting to create the illusion of that the network wishes for very large blocks.
Actually, my Sybil attack can go something like this:
1) Vote for 1.5 MB blocks
2) Check how many nodes I have killed with the increase
3) Vote for 2MB blocks
4) Check how many nodes I have killed with the increase
X) Keep doing until I am the network of nodes. Fill up the blocks and start pushing out the Chinese miners as well.
Incremental Sybil attack, cheap and easy to pull off.