I think it's a fair thing to bring up, given that the F1 abstract comes across as claiming your-cake-and-eating-it-too:

"With F1, we have built a novel hybrid system that combines the scalability, fault tolerance, transparent sharding, and cost benefits so far available only in “NoSQL” systems with the usability, familiarity, and transactional guarantees expected from an RDBMS."

In other words: sure, you can build a distributed, transactional system by making all writes synchronous... but most (all?) NoSQL systems prefer to optimize for low-latency operations over transactionality. As an author of one of those systems, I think that low latency is a more useful property in general, but the fact remains that it's a tradeoff, and it's a shame that the F1 authors deliberately gloss over that.