I understand one of the goals for Orion has to be a distinctive UX of it's own, along with other benefits over other browsers, and Safari in particular…
But personally I'm just not convinced by the Vertical Tabs approach — that's OK, I don't have to use it, except, unfortunately I do if I want the Bookmark Sidebar UI. I find that unfortunate, and somewhat counter to what I'd otherwise understood as Orion's goal of allowing user agency.
Further, I find myself disappointed by Safari's change to Bookmark Sidebar behaviour, linking it so closely to Tab Groups (which I've not started using, and not sure if I want to…?).
Previously in Safari one could show the Bookmarks Sidebar in a new blank window (i.e. alongside the 'top sites' feature), but then have it hidden when following a bookmark, i.e. the entire window contents was replaced with the site content. In Safari 15 they want to force me to have it shown all the time, which I don't want.
I had hoped I could still achieve this UX via Orion, but the current approach of inextricably linking the Bookmarks Sidebar to Vertical Tabs absolutely prevents that possible UX and feels even more restrictive than Safari.
I'll watch how the whole sidebar UX evolves, let along Vertical Tabs, and if see it might eventually become attractive (to me!) to use.
Just my 2c! 😀