If doing them on my Kickr, I prefer to use sim mode and it will feel more like a hill. You could raise your front wheel for a little more realism, and place hands on bars as if you were outside.
FWIW I’ve always done them on flat roads, along with high cadence work, and have had no issues doing actual long 1-2 hour HC climbs (I’m usually around 2.7-3.0W/kg).