Might sound unlikely but it happens.
Also, if github is down or your repo with them is corrupted, you have to go through their support. If your own server has a problem you can fix it instantly.
I'm not convinced that reliability is the correct reason to go GitHub. Features: Yes. Reliability: Not necessarily.
Data safety is all about fighting correlation. You don't back up one partition to another on the same spindle, because when the drive dies the whole spindle is lost. Paranoid people back up to two different drives, two different disk controllers, two different machines, two different datacenters, two different continents...
 But nonzero. It is worth thinking about the scenarios.
In addition to the remote repos on Github and my normal local copies...