How many string keys? Millions? Billions? What happens if you lose an update?

Redis great for lots of rapid reads and moderate write speed for data that fits on a single server or can be manually sharded well (if it's straight k=>v, as you describe, that basically means your json fits in RAM; if you're using larger objects like sets/zsets/etc, it becomes a slightly different discussion), as long as your application can lose a few seconds without killing you (BGSAVE isn't instantaneous, of course).