Talkray (YC W12), an under-the-radar messaging app, takes off overseas

I'm assuming they're just using a simple "store and send repeatedly until receipt is acknowledged" model, kind of like Skype does. Maybe I'm just naive but is it difficult to build a messaging app that works fine over unreliable connections, as long as a small delay is acceptable?

User interface and tie-in features seem like the only difficult parts for apps like these.

edit:

Nevermind, read more of the article and realized they also optimized voice calls for slow and unreliable connections, which is a far more difficult feat.