Manjaro sells itself as "Arch, but more approachable". In reality, you'll often end up with "Arch, but with additional weird package management upgrade issues that are a byproduct of Manjaro's own repositories interacting with the arch on your system."

Instead of just having to track the arch repos, you suddenly have those and Manjaro's own stuff (and own package manager tool etc.), which is another point of failure. Every new bit of technology is another part that can fail.