Substack fails on several points for me.
On the first or second page view of any particular blog, the platform likes to greet you with a modal dialog to subscribe to the newsletter, and you have to find and click the "No thanks" text to continue.
Once you're on a page with text content, the header bar disappears when you scroll downward but reappears when you scroll upward. I scroll a lot - in both directions - because I skim and jump around, not reading in a rigidly linear way. My scrolling behavior is perfectly fine on static/traditional pages. It interacts badly with Substack's "smart" header bar, whose animation constantly grabs my attention, and also it hides the text at the top of the page - which might be the very text I wanted to read if it wasn't being covered up by the "smart" header bar.