All true stuff. I'd like to add though, time based expiry is in many ways the best option if you can get away with it. If you can say "this can be 5 minutes out

All true stuff. I'd like to add though, time based expiry is in many ways the best option if you can get away with it. If you can say "this can be 5 minutes out of date" at a product level, a lot of cache complexity simply evaporates. Even versioning data, as you mentioned, is tricky. Once you add in a few joins ensuring the version is always correct can be hard.