Nice, Ueno seems to be sharing a lot of useful insights these days. In my studio we decided to go digital with all the guidelines we design a couple of years ago (with an occasional PDF or a printed book on client's request). That does make the process faster and assets easier to maintain for us but, to be honest, a bespoke book is still hard to beat if you really want to communicate a brand's character well. Bespoke website presenting the brand would be even better for character, communication, access and updating, but that's too expensive for most.
For our current beta guidelines system, we're custom coding based on Bulma framework which is still rather generic regardless of skinning although very quick to do. And free. (Some examples at: https://metroteka.com/brand/, https://clients.filburg.co/pileivile/brand/)
Online products (such as Brandpad) look quite generic as well so seem to be well suited for digital styleguides and assets, less suited for visual identity guidelines which usually need more presentation flexibility, and least well suited for brand guidelines which need much more narrative possibilities. Has anyone found/made a list of competing products in this space?