I wouldn't think the cross-scheduling issue is actually a huge practical problem, as it requires fairly specific failure cases. OTOH if the users are disconnect

I wouldn't think the cross-scheduling issue is actually a huge practical problem, as it requires fairly specific failure cases. OTOH if the users are disconnected, then you can't solve the problem anyhow, so you just write a consistency checker that runs while/after syncing and alerts users. I think the real problem is sheer engineering complexity: having to support a large number of input/output formats, protocols, devices, locales, languages, countries, timezones, platforms, legacy features, backward-compatibility, tools, etc. and making it all look good on the desktop, web and mobile app. I think the OP is saying if you multiply these together, then you're screwed, unless you're Microsoft or Google...