I never had to give a scathing code review but every time I had to give one with a lot of requests for changes I would always point out why I'm making each requ

I never had to give a scathing code review but every time I had to give one with a lot of requests for changes I would always point out why I'm making each request - with links to style guides, articles about code quality, code smells, best practices etc. There are also automated tools that can do some of that for you (codeclimate.com comes to mind) so that you can delegate some of your 'work' there. BTW from my experience teaching people to avoid nested control flow is one of the simplest things to do - just show them how many different code paths are possible (and how many tests would be required to cover each one) in a function with 4 nested if's.