The fundamental problems are communication lag and lack of information about why issues occur (encapsulated by the Byzantine Generals problem). I like to imagine trying to build a fault-tolerant, reliable system for the Solar System. Would the techniques we use today (retries, timeouts, etc) really be adequate given that lag is upwards of hours instead of milliseconds? But that's the crux of these systems, coordination (mostly) works because systems are close together (same board, at most same DC)