GitHub is a much larger target, and has many, many more moving parts than your average Gitlab on, say, a Linode virtual server.
Github will probably have more downtime which you can do noting about, than your selfhosted gitlab, which will probably fail because you upgraded something, or tinkered with a configuration.
It's a numbers game. They have more people monitoring, they spend way more money than you do on uptime/services, but such a service doesn't get more stable as it grows.
I trust myself way more than I trust GitHub or other hosted/cloud services because I cannot affect/help them in any way when stuff happens.
Ultimately, I know this, I accept this, and I happily use GitHub, Bitbucket and such all the time.
Github will probably have more downtime which you can do noting about, than your selfhosted gitlab, which will probably fail because you upgraded something, or tinkered with a configuration.
It's a numbers game. They have more people monitoring, they spend way more money than you do on uptime/services, but such a service doesn't get more stable as it grows.
I trust myself way more than I trust GitHub or other hosted/cloud services because I cannot affect/help them in any way when stuff happens.
Ultimately, I know this, I accept this, and I happily use GitHub, Bitbucket and such all the time.