When it is sized to the load the problem is it would only shut off for
an hour or two as the house cooled, then it would run steady until
after the sun came up to catch up. No real point in setting back.

The only way set back works with a heat pump is if it is grossly
oversized. All that accomplishes is set back without auxiliary heat. It
would short cycle inefficiently except for when it was trying to
recover from a set back.

In an environment with an ambient dewpoint above 60F there will be
problems with a grossly oversized system in cooling mode.