Many years of experience. When Apple releases a new iOS or OSX version, a clean install is, in my opinion, vastly, vastly superior to the incremental upgrade. No backups, no restore, no time machine -- a true clean install, followed by new app installs.
First, the iPhone/iPad/Mac operates as if it were new when I do a clean install. I never see any of the "sluggishness" or "slowdowns" that many people complain about with older devices. I can use an iPhone for five, six, seven years, and it feels brand new out of the box after a clean install. This is a testament to Apple's solid hardware/software integration and support -- indeed, this is the key reason we pay the Apple premium, because the products can be made to last a long time.
Second, almost no bugs, errors, or crashes after doing a clean install with a major OSX / iOS release. Again, users who incrementally update from one version to the next report issues, but I never see them when I'm doing a true clean wipe and restore.
I do incremental updates for the small releases, like security fixes and bug patches. But for any X.0 release, it's a clean install every single time, on every device. Yes, it takes time, but I'm always thrilled with the result, as well as the peace of mind that there is no detritus, permissions errors, driver problems, old app file conflicts, etc. -- all of that disappears with a fresh system install and downloading clean, updated apps. It's the only way to go, as I see it.