We're only more reliable when self hosting. GitLab.com is growing very fast and we have availability and speed problems. But I love the 'for now'. We're working to improve .com and we'll keep launching awesome features every month.
At least when it's self-hosted, it's your responsibility and you can fix it. When you are anything larger than a tiny company, this is much less risky.
Remember that 'the cloud' just means 'someone else's computers' so you are subject to their infrastructure management practices and uptime guarantees (or lack thereof).
Even for a company much larger than tiny, you're better off with GitHub from a reliability perspective. Most medium companies (up to hundreds of employees) do not have operations staff anywhere near as responsive as GitHub's are.
Look at their status history. The vast majority of companies could not boast such a record for their internal operations.