First let me say I have only the most cursory understanding of "the reordering problem". My assumption is that since CUSC is batching up sub-lists of the update set and sending them in parallel that if doc1 is followed by doc2 in the original list, doc2 might get to the indexing node before doc1, be it an update, delete, add, whatever.
That said, I don't really understand how reordering matters if (as per the original problem statement), it's guaranteed that each document is new and is submitted exactly once ever. I guess another important restriction is that the client doesn't care if docs get into the index in a different order than they were sent. How would correctness be threatened in that situation?
If the concern is that this is a too-specialized use-case that allows people to set it and shoot themselves in the foot too easily, that's a point. I just don't get why, in this specific use-case, this is a correctness question.
All that said, if Yonik's fingerprint stuff is going in relatively soon, it's probably all moot and we can just wait on this...