It depends on your needs and team size. For few tens of dev that need a versioning system with some sugar, a self hosted gitlab can acieve better uptime.
Gonna call BS on that.
It can achieve better uptime, if you ignore the downtime for upgrades, the downtime for configuration errors, the downtime when the disk fills up...
Btw, you cannot compare a scheduled maintenance window with an unplanned downtime ... maybe in the middle of an important deployment.
Bitbucket Server
Besides the last one, the others you can schedule when developers are not actually working on something, or give a headsup so developers can be prepared in case of errors.
In the case of GitHub, Microsoft deploys changes whenever they want, whenever you want it or not.
That's because GitHub is used by millions of people every day all over the world. Business hours for them are around the clock. No matter when they update, if it breaks something, many people will hear about it regardless of when the break was deployed.
So 10 minutes a month for upgrades, zero time for the rest because you are managing it just fine.