Well, it’s a little more complicated than that; some devices have channels that don’t receive values until their state changes (things like Z-wave devices). So if you add a z-wave device alarm sensor, the device will be online but the value will be UNDEF. You have to uses persistence and trigger the device on/off in order for the UNDEF to clear.
Aside from that caveat, UNDEF seems to be pretty reliable. Like I said, this is OpenHab setting it in response to thing transitioning to offline, so at least you don’t need to rely on binding authors to do this.
I’m wondering rather if an issue here is that the offline status isn’t surfaced through basic UI? It might be possible to do a conditional icon for UNDEF state in the sitemap, I don’t know, I’ve not tried it, but seems like sometime like that should be possible.