Feel free to file an issue if that's a concern -- if enough people feel the same way we could provide an option to "synchronous consistency". It wasn't one in the use cases we looked at.
It would be a significant performance tradeoff, if I understand correctly. So there's no point, there are enough ACID in-memory databases in the world)
Most likely not a performance tradeoff. For every Get and Set, we do lookups anyway. Caffeine, for comparison, always sets the key-value, and then if the key doesn't get admitted due to policy, deletes it later.