I'm guessing that people who have issues with the installation of 8.5.15 have, at some point, had their registries altered. This might be because of the use of registry cleaners, it might be a Windows update patch installation, another program etc.
What it isn't, I don't think, is an issue with the Steinberg installer. Why should it be? If the Steinberg installation registry entries are OK, then there won't be an issue. If they've been altered, deleted etc, then problems will happen.
I had to re-install 8.5 when 8.5.10 came out. It wouldn't install 8.5.10. My bad. I'd run a registry cleaner, not checked the findings and just clicked delete. Embarrassing? yes, lesson learnt? yes.
I know it's frustrating, but user errors, mistakes, misunderstandings account for the majority of problems down the line. I've done my share of them, believe me.