I think ROM refers to everything from the hardware interface to the bundled software in this case, rather than the technical definition of ROM (or, more appropriately, firmware).

For the most parts, removing bits and pieces of the system works well. Yet it does have some drawbacks. If you get a little too zealous, breakage is easy. Also, how many people understand what changes Kobo made to the system? The user-facing stuff is the obvious part. While poking around, it seems as though other modifications were made as well. Kobo's fingerprints are definitely on some of the initialization files (as an example).