Every real-world colocation or self-hosting project I've ever been around has underestimate their downtime and rate of problems by at least an order of magnitude. The amount of time lost to driving to the datacenter, waiting for replacement parts to arrive, and scrambling to patch over unexpected failure modes is always much higher than expected.
There is a false sense of security that comes in the early days of the project when you think you've gotten past the big issues and developed a system that's reliable enough. The real test is always 1-2 years later when teams have churned, systems have grown, and the initial enthusiasm for playing with hardware has given way to deep groans whenever the team has to draw straws to see who gets to debug the self-hosted server setup this time or, worse, drive to the datacenter again.