1) the more nodes, the higher probability of failure, 2) the more load, the higher probability of failure, 3) complex logic and/or nodes that aren't tested frequently have a higher probability of failure
Basically, every person who has claimed "X is easy, I do it for my personal use and it's fine with one giant node, don't need HA" is true (for them) because it's literally less likely to fail. But if they had more load they'd see more failures, and probability doesn't mean they won't encounter the random failures that are much harder to recover when there's no backup system.
HA means you're going to have more failure, which means more work to keep it running. But it's necessary work if total system failure is unacceptable. Two engines in an airplane is twice the maintenance, but it's better than the only engine conking out mid-flight.