Bitcoin Unlimited

Yeah, if his 1st block splits the network 50:50 then you can say the hardest part is done, and he can use his 5% to try maintain the instability until parking lock in, but there's still a good chance of this resolving before 4 blocks when either side gets a 2-block lead Tricky part is in even learning exact hashrate behind node IPs (although there is a method, that has been recently replicated but only managed to pin down 50% to 200 "influential" IPs: https://github.com/Crypt-iQ/crypt-iq.github.io/blob/master/coinscope-post.md#mempool-partitioning-and-identifying-mining-nodes) The other tricky part is in betting on hashpower not changing due to a race; miners who have skin in the game can add more hash to increase chances of victory, and attacker has no way of knowing how much either side has in reserve