Great guide if you are new to web servers. I run a very similar setup personally, but I use a remote Mac mini server instead of a Ubuntu server. Either way it works exceedingly well.
There is one caveat that wasn’t mentioned: latency.
When you pop open Dropbox on iOS it’s effectively ready to go, and pretty fast too. With the BitTorrent Sync app there is a long delay. On my iPhone 5s, over WiFi, it took 11.5 seconds before I could tap on any folder in the BT Sync app. That’s a long time if you are trying to be speedy.
That said, for privacy minded people, this is a worthwhile trade off.