Approximately 70% of the heat loss from your body is as radiated heat. Ceiling insulation is critical for thermal comfort. Have a look at the Efficiency Matrix YouTube channel videos on insulation consistency.
Less that 2% of insulation being out of place can waste something like 40% or some ridiculous amount of energy. It's quite nuts how even just a little break in insulation can have a significantly negative effect. A few years after I moved into my first home - a townhouse - I added some R-16 bats to the already existing R-12 insulation in the attic and it more than cut my heating bill in half. It was a three story narrow, townhouse - not that much square footage of ceiling compared to the walls/rest of the house but it made ALL the difference. If I didn't live it, I wouldn't have believed it. And I kind of did it on a whim since I didn't need that many and Lowes was having a sale on insulation. I would have done it the first week I moved in and had a couple far more comfortable winters!
It’s still a thermal path between inside and outside, and thus wastes energy if you’re trying to maintain an internal temperature that is different from the external temperature. Also there are radiative effects - hot spots on ceiling will radiate IR into your space. Just try standing under a corrugated steel roof in the summer.
Because in the winter the thermal energy within your room will “flow” out of those “cold spots” at surprisingly high rates; in summer those spots act as space heaters. In both scenarios they will adversely affect the ambient temp of the room. In short, whatever method you use to heat/cool the room will have to work harder and will take longer. Here’s [0] a good overview of heat flow in this context, with some hints towards the math behind if you wish to go deeper.
I read it as: Ducts are better but not so much that it's worth tearing everything apart to install them. If you don't already have ducts, you should definitely use a ductless system. If you do have them, you get some other benefits.
I think he means having a duct work for the whole house for a central heat pump is good. But having a heat pump per zone is also good if you don't have ducts for the whole house. Some newer home designs separate heating and cooling from the air exchange systems so there are multiple zones of heating and cooling.