FYI, I think you are talking about the type that mounts on top of the strut. These will allow excessive droop. Excessive droop can lead to over extension of the CV and they can actually pull apart. This can also cause extreme CV angles, at full droop, that can cause CV failure.
Coils spacers.
There is zero proof that they cause coils to sag prematurely. Nor do they cause a harsh ride.
The front spacers, that are mounted in the coil/shock assembly, do extend the shock and the shock now is working toward the end of the shock. Nothing different from a longer coil.
Rear spacers stack on top of the coil. This does nothing to the coil. There are no new forces on the coils. They only raise the vehicle like a longer coil.
Most of the time spacers are used on 10-15 year old coils. A coil this old is prone to sagging. It is more of coincidence that they sag more after adding spacers due to age and not from the spacers.