The simplest devices are on or off. If they are grossly oversized they cycle a lot and make homes less comfortable by pushing a huge volume of hot or cold air briefly.

Variable speed compressors are better, but blowers may not also be variable speed. So you'll get better efficiency but may still suffer a feeling of draftiness.

A properly sized variable speed unit will operate within some optimal band of efficiency and constantly output nearly the minimum necessary air volume to achieve the target temperature.