Heater cores aren't used that often but perform relatively well because they are very similar in design to watercooling radiators. There are additional considerations when choosing a heater core over a normal watercooling radiator:
1. You have to fit some kind of fitting as the existing fittings might not allow tubing connection
2. You have to consider how to mount fans and then mount the heater core in or on the case
3. Some heater cores are aluminum, so you'd want to avoid them
4. Heater cores come in many sizes based on automotive needs, but in reference to the DD core you're discussing you still need to determine total loop TDP.
5. Heater cores are designed to dissipate much higher heat loads (automotive coolant at operating temps around 160-180F) so the wall thickness of the tubes and overall structure are much thicker. This means your lower water temps of a watercooling loop aren't less effective when dissipating in comparison to the very thin walls of a watercooling rad.