I booked a hotel this morning. My observation is it depends on the hotel price. If the price is less than rm 300, there is a mark up (ex. rm 173 with Agoda only, rm 180 with Shopback). One hotel has rm 301 per night and the price is still the same with Shopback.

Edit: After I compute, you still pay less with Shopback but it is not always 6%. If less than rm 300, the effective cashback rate can be 2.5% because of the mark up.