(Not OP) The kind of regressions I'd think of would be things like:

- updating the URL state when someone clicks on a button

- proper back-button support in the browser that takes me back to the prior 'page'

- being able to navigate to any URL deep in your app and get a valid response (ideally rendered server-side so there's no client-side loading delay).

Things like these are hard, and the reason why it's common advice to use a framework and not hand roll. If you hand roll but don't support these things gracefully, you're making a case for not hand rolling.