Any thoughts on this paper https://arxiv.org/pdf/1612.09426.pdf ?

It describes a "Balance Attack" against bitcoin and ethereum where you split the miners into 2 equal groups by causing a communications delay between them. Then you mine with one group while submitting transactions (for merchandise, services, etc) to nodes connected only to the other group (and you can send same coins to yourself on your mining branch). When the communications delay is lifted the chain being worked on by the group you were mining with should be longer than the other group's chain, so consensus will then cause your spent merchandise transactions to get undone, while spends to yourself from your mined chain will persist. Thus a double-spend.

Authors were focused on Ethereum and private R3 implementation but they also think their work extends to Bitcoin as well.

Why do miners have to be evenly split to start with?  Also, they use "communications delay" but for bitcoin, this would be more of a disruption between the 2 groups right? Also the need to isolate the node you are purchasing goods from to one side.

So definitely a few challenges pulling it off. Still with small number of mining pools controlling most of hash rate, it doesn't seem crazy that they could be split into 2 groups.