While I know what you're getting at, the word "never" really isn't helpful or useful. There are certain times where innerHTML(or insertAdjacentHTML) is the best approach. It's really not that slow that it can't be the most clear and concise solution.
Its not about performance. innerHTML is actually fast. Its about security and maintainability. InnerHTML makes it easy not only for the developer but also an hacker to inject code into a web page/app. innerHTML is like a global missile satellite network that can fire anywhere at any time. Instead you want to design like a plumber, where data flows via a pipe to the database, then back again in another pipe which change the state of the view. You should not have a single HTML tag in your single page application!
