When landing flaps 40 on a B737 reducing to idle before descent is stopped is always an absolute disaster unless you have a lot of excess airspeed.
Not necessarily. If you are at bugged speed & commence the thrust reduction at about 30' to reach the idle stops by touchdown &
flare the aircraft, you will get a smooth, nose high touchdown, as you should. If you carry the power into the flare & then chop it you will land very flat. If you are also too fast in this scenario, you will float & risk the possibility of a nose gear first touchdown (also know as wheel-barrowing.)
The main difference between flap 30 & 40 is that the flare needs to be more positive with flap 40 to get the nose high. To do that you need to have the power off & airspeed reducing towards Vref in order to prevent ballooning.