It doesn't have undeletable apps, since adb lets you delete anything (although the exact semantics of "delete" will be different for apps that are installed on the system partition - but not in a way that matters to end users).

It has apps that cannot be installed from the phone UI. Which kinda makes sense, since you don't want people randomly uninstalling, say, the app store, and then complaining that their phone is bricked. OTOH, someone who knows how to do this via adb can be presumed to know what they're doing.