The caution is to be aware of the spool up time required in case of a go-around. Obviously differs with engine type. What Vref do you bug up in a 737 for a flapless landing? As it's not supposed to happen, Boeing did not have flapless landing drill in the QRH for the Classic 747 but we did practice one in the sim during our conversion course in Seattle and the bug setting was Vref, flap30+40kts.