You're correct in your assessment of the worst-case of distro maintainers, however many distro developers/maintainers do contribute to the upstream ( Debian policy explicitly encourages it, I only speak for Debian because that's the only project I've worked in) and do vet and understand the software.
"It can't be better". Except distro maintainers can block it from being included if they find errors. As noted above they also often file pull requests against the upstream. This happens a fair amount actually.