- A large number of users will increase the likelihood that one of them will encounter a temporary issue; even if problems are intermittent, a single person using the system simply won't notice them, but lots of people will.
- More users tends to mean using more features, more system resources, etc, which increases the likelihood of an issue from either buggy/complex features, or resource exhaustion.
- Updates for a team typically involve being more careful and performing updates on a regular schedule, in order to minimize downtime.
- If a problem does occur and you're not around, the team is stuck until you are available. If you go on vacation or are hit by a bus, they're stuck longer.
- Having more users tends to require things like disaster recovery contingencies, security, etc. Somebody has to do this extra work, which is a cost in time and labor.
Where self hosted shines is in keeping complexity down and changes minimal. GitHub is a giant system, making it more likely for problems to interrupt service. They constantly ship changes, increasing the likelihood of interrupted service. A self hosted option can use the same simple system and version for much longer, only taking security patches until the version is EOL and needs to be upgraded.