South Australia went and installed one of the aforementioned monolithic batteries. It has actually kept the grid synchronised very well, once responding in much less than a second, keeping the grid stable even when a 600MW generator fell off a cliff. The one advantage of this approach is, even though it is a complex system, it is still not as difficult to orchestrate as a bunch of independent, latency seperated smaller batteries. That isn't to say it is the only or best solution, but it is viable currently and immediately.