For example, most books in the kobo store have DRM (to be fair I don’t think it’s entirely their choice), albeit an easy one to break.

You see people here recommending “get a Kobo and install koreader”. That works and there is a one-click install process but it’s still an hack.

Another common thing to do is to keep the stock reader but tweak a file to point the store API to a self-hosted Komga/Calibre Web Automated/Grimmmory instance to get your own books in the system. Again, it works well enough but it’s not like these are documented API.

The bottom line is: it works very well and you can very easily tweak it but the way you tweak it are hacks (reasonable clean ones) rather than officially supported features. Thats why it loses a few marks