The inability to change settings or sync data from a local device connected via Bluetooth is a huge flaw. The inability to view historical data in the app is more understandable but still a major shortcoming, especially since mine appeared to be cached until the last cloud update timed out and the cached data disappeared.

I'm fairly certain memory isn't an issue for my devices to continue collecting data, but there's no good reason to prevent Bluetooth comms between phone and watch when the internet is unreachable.

source: I use a Forerunner 945 for fitness & all-day tracking, and a Vivosmart 4 for sleep tracking